Are the tracks that this happens on ones that you have converted from GTR2? The reason I ask is that some people don't delete certain lines from the AIW files which are for garages and maybe that is the cause.
The lines that need to be deleted (not always apparent on every track) extract from tutorial thanks to N2LO:
) Open up the track .AIW file and delete the following entries
Under the [Grid] (X= numerals) GridIndex=103
and under [Pits] Delete
The only other thing would be to have less cars on the grid so there are enough garage spaces.
I hope this helps you Dylan m8.