The blue PS2 disc's are CDs, the other ones are DVDs. And I have never had this probelm myself, but from what I have read etc. it seems, that things like that happen, when the Lense of the laser gets dirty. You can check this, if you take a music CD and test if it works. If it works, then the lense shouldbe clean and there is something wrong with you PS2 and if not, it just means you need to get the lense cleaned.