DVD Player reads CDs, not DVDs

OK, this might be more of a general hardware rather general pc question, but anyway :D

The DVD player my parents have in their living room used to work fine, playing DVDs, VCDs and Audio CDs. Gradually, it started reading only some DVDs and some of them it simply refused to read...

Now, it won't recognize any DVD and it just prints a message "Please insert a playable disk". It still reads Audio CDs and VCDs with no problem whatsoever.

Any clues?

Gradually... are the DVDs becoming dirty? Are you experimenting with the same DVDs each time or different ones?

I haven't been trying with the same DVDs all the time. Yesterday I tried with 2 new ones and an old one...

dvd players that read cds actually have two different lasers in them. You probably need to use a cleaning disk in it. The lens that receives the image from the dvd laser is dirty.