Narrative:

Aircraft X; a phx departure was climbing out on the snobl departure climbing to FL360. Due to a ZLA LOA requirement; I also had 4 klas arrivals descending to FL300. I noticed that aircraft X was an issue with the last of the klas arrivals; and turned aircraft X 15 degrees right; to fall behind. During the shift we had bad rides anywhere from light chop to moderate chop to moderate turbulence. Aircraft Y who was further east entered the airspace at FL340. Halfway through the sector (about 5 minutes prior to turning aircraft X); aircraft Y asked for a better altitude; I had then stated that I no complaints at FL300. Aircraft Y asked for FL300 and I descended the aircraft.while aircraft X began his turn; I noticed that aircraft Y was also traffic; and was planning to get aircraft X on course at the earliest time. I cleared aircraft X on course; and had he done the turn right there; this would be a non-issue. Aircraft X slopped his turn (possibly due to the winds) and I recognized that I was not going to have separation with the current status (just slightly over 2 minutes out). I called ZLA and appreqed aircraft Y descending to FL280 for traffic; they said approved. I keyed up to issues the descent; 'aircraft Y descend and maintain FL280 for traffic.' I believe I issued a 2;000 feet per minute descent rate (I know this is not 'valid' but I know that it helps convey a rate needed; and is used as an advisory). I waited; and received no response. I keyed up and said the same thing; then waited; no response. I keyed up a 3rd time and issued the descent. After the 3rd call; aircraft Y asked if I was calling him. I said yes; then issued the clearance for the 4th time. The pilot waited a second; then responded with; 'can you confirm the altitude for aircraft Y?' I reissued the clearance; and told the aircraft to expedite and that I needed him 'out of 29 in a minute of less.' aircraft Y waited a second; then read back the clearance. I then told aircraft X to expedite his climb and to report out of FL310. At that point; aircraft Y said that he had the traffic in sight; 'traffic in sight 6 miles in front 400 feet above.' and aircraft X on his own said he had the 4 o'clock traffic. Aircraft X was at FL305 climbing; aircraft Y level at FL300; 2 hits later; I lost separation at circa 4.8 miles; the next hit was around 4 miles; I saw as close as 3.75 nm between the targets.all the time; ZLA had the hand off on aircraft Y; after appreqing the descent; ZLA never changed the data block. I know that around 3.75 miles I had 1;000+ feet; because I finally saw aircraft Y out of FL295. But I cannot tell you what I really had. After aircraft X was out of FL295; ZLA changed the data block to reflect FL280.I firmly believe that I would have had separation had aircraft Y responded to the first clearance. It took 5 transmissions to get the clearance to aircraft Y (in my opinion that is unacceptable). My back up plan was to descend aircraft Y. I went to execute the backup plan and aircraft Y didn't listen. I really feel that I let the ATC system down. I trusted that the pilots would listen and follow directions. I have had a handful of close calls in my career; but up to this point; I maintained separation because the pilots responded on the first call. This situation really caught me off guard. I didn't expect so much effort to relay a descent clearance to aircraft Y. I am truly sorry. I could have stopped aircraft X at FL280 and never climbed him above the traffic. But I was trying to provide the best service possible. And providing that service caught me.having pilots that listen. Change data blocks when altitudes are appreqed.

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Original NASA ASRS Text

Title: ZAB Controller describes a loss of separation he attributes to one of the aircraft taking five transmissions to receive information.

Narrative: Aircraft X; a PHX departure was climbing out on the SNOBL departure climbing to FL360. Due to a ZLA LOA requirement; I also had 4 KLAS arrivals descending to FL300. I noticed that Aircraft X was an issue with the last of the KLAS arrivals; and turned Aircraft X 15 degrees right; to fall behind. During the shift we had bad rides anywhere from light chop to moderate chop to moderate turbulence. Aircraft Y who was further east entered the airspace at FL340. Halfway through the sector (about 5 minutes prior to turning Aircraft X); Aircraft Y asked for a better altitude; I had then stated that I no complaints at FL300. Aircraft Y asked for FL300 and I descended the aircraft.While Aircraft X began his turn; I noticed that Aircraft Y was also traffic; and was planning to get Aircraft X on course at the earliest time. I cleared Aircraft X on course; and had he done the turn right there; this would be a non-issue. Aircraft X slopped his turn (possibly due to the winds) and I recognized that I was not going to have separation with the current status (just slightly over 2 minutes out). I called ZLA and APPREQED Aircraft Y descending to FL280 for traffic; they said approved. I keyed up to issues the descent; 'Aircraft Y descend and maintain FL280 for traffic.' I believe I issued a 2;000 feet per minute descent rate (I know this is not 'valid' but I know that it helps convey a rate needed; and is used as an advisory). I waited; and received no response. I keyed up and said the same thing; then waited; no response. I keyed up a 3rd time and issued the descent. After the 3rd call; Aircraft Y asked if I was calling him. I said yes; then issued the clearance for the 4th time. The pilot waited a second; then responded with; 'Can you confirm the altitude for Aircraft Y?' I reissued the clearance; and told the aircraft to expedite and that I needed him 'Out of 29 in a minute of less.' Aircraft Y waited a second; then read back the clearance. I then told Aircraft X to expedite his climb and to report out of FL310. At that point; Aircraft Y said that he had the traffic in sight; 'Traffic in sight 6 miles in front 400 feet above.' And Aircraft X on his own said he had the 4 o'clock traffic. Aircraft X was at FL305 climbing; Aircraft Y level at FL300; 2 hits later; I lost separation at circa 4.8 miles; the next hit was around 4 miles; I saw as close as 3.75 nm between the targets.All the time; ZLA had the hand off on Aircraft Y; after APPREQING the descent; ZLA never changed the data block. I know that around 3.75 miles I had 1;000+ feet; because I finally saw Aircraft Y out of FL295. But I cannot tell you what I really had. After Aircraft X was out of FL295; ZLA changed the data block to reflect FL280.I firmly believe that I would have had separation had Aircraft Y responded to the first clearance. It took 5 transmissions to get the clearance to Aircraft Y (in my opinion that is unacceptable). My back up plan was to descend Aircraft Y. I went to execute the backup plan and Aircraft Y didn't listen. I really feel that I let the ATC system down. I trusted that the pilots would listen and follow directions. I have had a handful of close calls in my career; but up to this point; I maintained separation because the pilots responded on the first call. This situation really caught me off guard. I didn't expect so much effort to relay a descent clearance to Aircraft Y. I am truly sorry. I could have stopped Aircraft X at FL280 and never climbed him above the traffic. But I was trying to provide the best service possible. And providing that service caught me.Having pilots that listen. Change data blocks when altitudes are APPREQED.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.