I've encountered the same issue with some of my 400k drives. I'd recommend not running any more disks through until you have a fix, as the plastic head sometimes can scratch the media surface of the floppies. I've had some luck in the past taking a felt pad (like you'd put on the bottom of chair legs to keep them from scratching the floor), thinning it down and cutting it down to size, and glueing it on. It may not be very permanent, but it's the best fix I've been able to conjure up so far.