I basically purchased an external USB drive for my Win7 machine.
I'm guessing you have the newer plastic HD disks and are using them on the lower capacity Amiga drives. Kinda hit and miss to be honest. Try and get some DD disks from second hand older stuff off Ebay. That's assuming you don't already have some DDs.
Be very careful with the floppy drives not to muck around with the drive head worm. See this blog of mine where I mend an internal floppy drive for the 1200.
Mainly I just get a disk drive cleaner off Ebay and that does the trick.
The fact that it is happening on all your Amigas points to the media and not the machines. Have you got Workbench disks and do those work.
Try and get an external floppy drive and see if that works better. Really would be so much bad luck having that many drives fail. I get the odd one that struggles but would never anticipate so many not working.
http://www.scuzzscink.com/amiga/scuzzblog_september18/scuzzblogdseptember18_0401.htmhttp://www.scuzzscink.com/amiga/scuzzblog_september18/scuzzblogdseptember18_0402.htmscuzz
