For the A1200 - take the top off the disk drive and check that there are no foreign 'bodies' inside. I once had a similar problem and found the cause to be a large moth that had crawled inside and got squished in the read/write head assembly. Basically, the moth prevented the heads returning to the boot track - hence no boot.
For the A500 - if cleaning the drive heads doesn't work, then check that the floppy media within the disk is free to rotate and there's no physical damage (scratches, corrosion, etc.) If the disk is difficult to rotate, then try 'flexing' the outer casing to free it off a little. It's worked for me on several occasions.
