']I am quite, quite sure that what you're experiencing is not degradation, but physical damage, dust, dirt, etc. Make sure, under a bright artificial light, the the discs are SPOTLESS before scanning, and use canned air if needed.
I agree that this is by far the most likely explanation.
I find that it is very difficult to get a scan that looks as good as the original scan once I have handled the disc a few times, because dust clings to the disc surface the first chance it gets, and tiny scratches are very difficult to avoid.
Looking at the disc in normal ambient light won't really show you much - you need to have a strong direct (artifical) light source in order to really see if the disc surface is clean!