Trying to remember when I had one, from memory there is a plastic cog under the playfield somewhere, it split on one side and although I couldn't see it it caused the copter to stop moving.
Look at page 75 of the manual (lever and pivot arm assembly) I think it was part 5B.
Also while it is probably pretty obvious have you checked the friction wheel (part 4A)? This often wears out.
Cheers