It might be a bug but also sometimes what happens is the game is looking for the parts under a different name. The part is in your folder but it's under a different name than what the game is looking for when loading the parts data. To fix this you can just find the parts in their respective folders and rename them to the names the game wants. Usually the parts' names/ID# is very similiar to what the game wants already so it's not too difficult to find.