Some CD's won't play... What's the problem?

My Sandling CD-T100 has trouble playing some CD's. The drive is trying really hard to read the disc (which is audible), and then it gives up. I get a "No Disc" on the display. Any opinion/advice?
Thanks you!
Store bought commercial CDs or did you burn them?

The usual culprit is the laser is out of alignment if it can't read store bought commerical CDs. Could also be dirt on the lense, but less likely. Burned CDs are sometimes hard to read, and you could also have a bad or misaligned laser on the burner as well.