B737 captain reported that an aircraft failed to vacate the runway; resulting in a go around.

Attributes
ACN	1712778
Time
Date	201912
Local Time Of Day	1801-2400
Place
Locale Reference	DEN.Tower
State Reference	CO
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Landing
Route In Use	Vectors
Flight Plan	IFR
Aircraft 2
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Landing
Flight Plan	IFR
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 95 Flight Crew Total 611 Flight Crew Type 611
Events
Anomaly	ATC Issue All Types Conflict Ground Conflict Less Severe Deviation - Procedural Published Material / Policy Ground Incursion Runway

Narrative:

We were cleared for an ILS to runway 16R. Spacing was 5 miles behind aircraft Y. ATC assigned a speed reduction to 170 kts. A further speed reduction to 150 kts was assigned. Traffic separation was 3 miles. Our target speed was 152 kts. We were cleared to land. I saw aircraft Y fail to make the 2nd high speed exit and informed the first officer that 'it was not looking good'. I started the go around at approximately 150 ft. The first officer radioed the go around to tower. There was a significant pause before ATC responded and they seemed surprised we were going around. We over flew aircraft Y on runway 16R. We were vectored around for a visual approach to runway 16L. A normal landing ensued. After exiting the runway; ATC informed us there would be an investigation launched regarding the go around. My assumption is that the controller forgot about aircraft Y on the runway which is why there was confusion as to what we were going around. Aircraft Y rolled long for unknown reasons and likely caught the controller off guard due to an unexpected maneuver.

Original NASA ASRS Text

Title: B737 Captain reported that an aircraft failed to vacate the runway; resulting in a go around.

Narrative: We were cleared for an ILS to Runway 16R. Spacing was 5 miles behind Aircraft Y. ATC assigned a speed reduction to 170 kts. A further speed reduction to 150 kts was assigned. Traffic separation was 3 miles. Our target speed was 152 kts. We were cleared to land. I saw Aircraft Y fail to make the 2nd high speed exit and informed the First Officer that 'it was not looking good'. I started the go around at approximately 150 ft. The First Officer radioed the go around to Tower. There was a significant pause before ATC responded and they seemed surprised we were going around. We over flew Aircraft Y on Runway 16R. We were vectored around for a visual approach to Runway 16L. A normal landing ensued. After exiting the runway; ATC informed us there would be an investigation launched regarding the go around. My assumption is that the controller forgot about Aircraft Y on the runway which is why there was confusion as to what we were going around. Aircraft Y rolled long for unknown reasons and likely caught the Controller off guard due to an unexpected maneuver.

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.