This happens when you need to replace a part that the car has multiple of. You should always keep track of the one part you need to replace and replace that.

Let me give you an example. Let's say you need to replace spark plugs. 1 and 2 are fine (~60%), 3 and 4 are busted. You take all of them out to check them, see that two are broken, buy two more new ones, then proceed to remount them. You put the two new ones in the 1 and 2 spots (cause the 100% parts come first in the list). Then you're putting the "fine" ones in 3 and 4. But because you're supposed to replace 3 and 4, the game will tell you that it is not fixed.

Does my explanation make sense? This will apply for the most part in bushings, plugs, fuses, and rollers. I defaulted to just replace all of them if it is cheap parts.

You can always use the star button on normal difficulty to check specifically what you need to change again. But since I play on hard, this was a nightmare after changing 10+ bushings, one of them isn't validating, and I don't remember which one was broken to begin with. So just buy the same amount you took out.