angle the disk (non label side) into the light and check for scratches, fingerprints etc. if you see anything, using a soft cloth ( old teeshirt material is perfect ) wipe the disk from the hole to the edge gently. go round the disk, but avoid creating a spiral.

last ditch rescue (and this really works) coat the nonlabel side in toothpaste and then gently polish the disk and rinse then pat dry with kitchen towel. I've done that many times to rescue disks...