This is usually caused by a failing DVD laser but in some cases can also be due to the laser not returning to the park position and thus getting stuck at the end of the rail. This issue usually only happens to the lite-on 74850C series drives, but may also happen to other drives. There is an easy fix if this is the case:
Get some 1-2mm plastic washers and cut 1 or 2 perfectly in half
Glue the 2 halves together in the half doughnut shape
Glue this piece at the end of the rail on the inner side like so (-
It's hard to explain in text but you just want the washer/s to stop the laser from going all the way to the end of the rail.
If anyone would like me to fix this issue for them, check out my site here: