I agree with Mr Logic. Some horses can work the track down but find it very difficult to produce the same effort in the pressure situation of a race. During trackwork, quite often they are either working on their own or with only one or two other horses. In a race they can become intimidated by the others and the pressure exerted.
I owned and trained two horse about 15 years ago and they were full brother and sister. Both worked well on the training track but only one of them reproduced that form on the racetrack. The better one went on to win both in the city and on provincial tracks while with the other I realised after two starts that it did not have the same ability and interest in a race and we gave up on her.
I have seen many horses over the years with heaps of ability but only the ones with that bit extra succeed.