I've found several:
* The rubber roller is not grippy enough anymore so it slips on the axle
* The drive belt to the roller is old and slips when there's too much friction, say a CD is inserted 😄
* The roller is super gross and too sticky
* The chassis is bent and won't press the roller against the CD anymore. I don't know how this happens but I've seen two.
Most likely cleaning the roller and replacing the belt will fix the drive