In 50% of cases I've diagnosed, the lens needed cleaning (esp. if you play the PS2 horizontally).
In the other 50 % there are a variety of reasons any/all of which could apply bearing in mind that the DVD side is much more sensitive than the CD operation:
1 You have a NEO2.2 and the constant voltage on pin 2 has worn the laser
2 The laser is worn out through having a difficult time with DVD-Rs
3 You have not calibrated correctly
This last point is important. If you hadn't cleaned the lens with a CD cleaner (flail disk), then your approach to turning the pots is crucial. If you turned the DVD pot up too far, you will have overloaded the laser and DVDs will never work again. If you did not use an ohm meter, then you won't know what to set it back to. Moving it up 10% on the original value is the only option in my experience; if it still doesn't work you need a new laser.
If you are in the UK you can get one within 24 hours from
www.kanection.co,uk for £45. DOn't know other places in the world apart form
www.lik-sang.com.
Finally do see my post entitled
Laser (Azimuth) Adjustment.