That's odd. Which platform are you playing on (Win/OSX/Linux/FreeBSD)? can you look in the vdrift/data/cars directory? Do you see lots of subdirectories there or just the few cars you had before?
OK, so when you go into the game, and click "Practice Game" you only can select 5 cars? Does the selector at the top have all 28 entries, and some of the cars don't show up, or it only has 5 entries? Can you tell me what the entries are? You can tell if you're at the end of the car selections because the arrow on the car selector should become lighter.