Does it make any funny noises like it's trying to or does it just flat-out refuse to play them? Any error messages? If it makes odd noises, chances are the bar the laser thingamagig slides on needs to be lubricated (carefully.) If it doesn't make noises, chances are you need to open it up and clean the laser.
Sometimes the laser gets too dirty for it to be able to read things properly. But if it's neither of those things, there's a chance your laser might just be getting too weak to read certain things. Then you're probably going to have to replace the laser altogether. :\