Who can you think that was/is a great runner, but when it came down to it, couldn't perform on the day it mattered? And conversely, who doesn't have consistently good races but can make it happen when something's on the line?
I can think of one off the top of my head.
Alan Webb - Wins 2004 Olympic trials in 1500, gets eliminated first round in Athens. Wins 2007 US Title in 1500, bombs World finals in 8th place. Didn't even qualify for 2008 Olympics. Sort of been a wreck ever since but seems to be coming back.