Best to try a different floppy boot disk. I find that mine are "going off" these days, i.e., they seem to work erratically: in one machine but not another, etc.

If booting with the new disk fails, it could be the floppy disk drive itself that is at fault. If it's been in the attic, etc., it may need a little tlc. I seem to recall reading instructions on how to rejuvenate them somewhere out there in cyberspace.