When you click on the car does it say you need to re-download or install it? If so go to the PS3 store or your account and look for downloads and then download the car.
I recently had this problem with the GT-R Stealth GT500 and it worked for me.
If that isn't your problem post what happens when you click those cars and post which cars have the problem.
if it's just a dlc issue re-download the car packs, however there is a bug where non dlc cars can also require a download to use (i.e the yellow exclamation you speak of) if this is your issue sell and repurchase the effected cars and that should fix it