Well, I used to flip back the disk door on some disks to see the state of the disk surface. Sometimes if disks have been stored in damp conditions they can accumulate condensation inside. Or if they come from an unknown source you can find them full of all sorts of dust and gunk.
Servicing a drive is pretty easy, most of the read/write issues I had was when something had fouled the heads, pull shell off the drive and inspect the heads for contamination, gently clean them with cotton bud and iso alcohol.
Other than that, it could be that the drives are just old. You could try to repair them by tracking down electronic faults such as dried electrolytic capacitors or lubricating the drive mechanism but Ive fount this to be very hit and miss. My A1200 worked fine when I put it into storage but when I took it out two years later the drive would no longer read disks, I tried to fix it but in the end it was easier to just replace it.
Also, I noticed my disk collection slowly eroding as time went on too, some disks that had worked fine for years would suddenly show errors. I put this down to disk aging.
