If the floppy disks you are using are High Density (You said they were new) then, it's possible that the new drive in the A500 is a High Density drive and can't read the disks because they are written in Double Density.
Put tape over the little window, to force the drive to read it as Double Density.
In short have a good look at the floppy disks that work and compare them with the disks that don't. If the disks that work, do not have the little window, then that's the problem.
Tape up the window on the disks that don't work.
