Many times, it is something simple. First, are these all "Real" CD's, or burned copies from a computer? (Not all CD Players will recognize do it yourself CD's). Then, check to make sure that they were loaded 'Label pointing up' (People do sometimes forget that). Then does the unit work in all other modes (Tape, Phono, Radio). Check each CD for scratches, Dirt, Fingerprints, Smudges. Then run a Cleaner CD Through it, such as the Scotch/3M 'EZ-Clean' which not only cleans it, but also tests it for Left/Right and surround sound, as well as cleanliness/purity of audio signal.

Good Luck, John Jewkes.