No disc error only for DVDs

I have a DVD player based on Sony Technology (bought 4-5 yrs ago) which is not reading DVDs (only DVDs). Earlier it used to read DVDs after so much of hit and trail and when the probability became too low, I changed that lens with a new one (both of 65 no). First day it worked fine but then it didnt read any type of disc which I found was because the discs weren't spin which I managed to get worked after recoonecting the bus cords but then after some time though everything was fine with the spin motor and the lens (because I get it checked by the retailer where It worked fine), it is giving no disc error for all DVDs AFTER READING IT for qite some time.

Now I wnat to know that how can I get DVDs work fine on my DVD player since there is no problem with the lens assembly.

Pls tell me what could be the other problems.

Or how to reset to factory defaults.

Also it is an assembeled DVD player with Tape Recorder and FM tuner with amplifier.