Either your floppy drive is starting to fail or you have some other problem.
If you can't replace your floppy drive then try cleaning it.
1) Blow it through with compressed air/canned air or use a bicycle pump.
2) Try a floppy head cleaning diskette if you can find one - they used to be common and commonly used - 50/50 isopropyl alcohol or meths (vodka at a pinch) and distilled water to dampen the pad.
Try using a different floppy formatting program such as the freeware
Format144 -
http://www.programurl.com/format144.htm - there are lots of others available for free on the Web.
Let us know how you get on!