I think it's just connected to the age of the machine, with lasers wearing out or getting misaligned over time. I read somewhere that standing the PS2 up on it's side rather than laying it flat helps, worked rather well when mine was having trouble with blue discs.
as above open it up clean the laser and adjust the caddy and focus a little and it will read them fine
I've done quite a few original PHAT consoles now and they all work great or better when they are adjusted
just be careful though when you remove the top part of the casing of the ribbon cable that goes from the top half of the casing for eject and power buttons to the board if it comes off the board be prepared for a mess about either taking it apart more or fiddling to get the ribbon cable reconnected without taking anything else off!
I bought my first PS2 when it first launched in Japan. Paid a shitload of money to be one of the first (At least in Sweden) to have a machine. You think i was mad that after a modchip was released that actualy worked, i found out that my laser was borked and DVD games refused to boot.
Most of their earlier machines have shitty lasers, nay, nearly all PS2 have shitty lasers. If you use cheap media or use the machine for a prolonged time, you are killing it slowly. FFS, when they released the Slim version the lasers actually melted when people played GTA - SA or any other disc intensive game for more then a couple of hours.
My PHAT PS2 doesn't read CD's at all, only DVD's. I don't care though, I have a ps1 for playing ps1 games, and CD player for playing music. The small percentage of CD-ROM ps2 games either suck, or can be put onto a DVD anyway. Oddly enough, a lot of the PS2 CD-ROM games also came on DVD's officially. Like with the original PS2 sims game, some copies were DVD, some CD-ROM