Does anyone else have problems with discs loading on a DVD player?

[PigglyWiggly]

I have certain discs that don't load. It either says no disc or tries to load but then rejects. The discs themselves seem fine and the player always plays most discs. It's just a few of them. Some do load eventually after several attempts.
I'm just wondering what causes it. It's annoying when you want to watch a certain movie and takes ages to load, or won't load at all when the disc seems fine.

 

[classicsat]

The laser wears out, or the laser block is dirty inside or on the lens. It could also be a "weak" DVD.