I recently had a similar problem and it was IMPOSSIBLE to find help so here goes...
From what I understand the CDs and DVDs use different lasers (dont know if this is true, I read it somewhere) so even though one may be functional, the other may not.
Easiest fix is using a disc-type cleaner. I did, however, read that these will damage the laser head over time.
Unfortunately, this didn't work for me so I had to physically take the drive out of the comp. open it up (voiding my warranty, of course) and cleaning the laser thing with alcohol (the high-grade kind but I think rubbing alcohol would work too) and a qtip.
Make sure you are VERY gentle! Do NOT apply more pressure than just the feeling that you are in contact with the lens! Otherwise you can damage the lens surface.
After cleaning it, dry it immediately with a new cotton tip or the other cotton edge.
That worked for me and hope this helps you too!
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