What I think you'll find is happening is that the screws that you are using are way too long or more likely when you tighten them up you are twisting the Floppy drive so it no longer works. I've seen this quite a lot in the cheaper cases where they are not square and actually twist drives around when fully tightened.
You only really need 2 screws to mount the floppy drive and try to use the really short ones like those used to mount Optical Drives. If your case has the ability to place a screw into a group of holes use them instead of the long slots that are commonly used and do not tighten up the screws just have them sort of loose but up against the casing. If you don't have that option then just tighten up the screws on one side of the drive and leave out any screws on the other side or maybe you can place one in on the other side if the drive is falling down slightly but do not do it up tight just use it to locate the drive in the right position so it is level. This might make the drive closer to one side of its opening than the other but this is not important provided that the floppy is working it's much more important to have a working unit than a cosmetically good looking case. If this doesn't work or drives you nuts with the extra gap on one side and you have a spare 5.25 inch drive bay available use a 3.5 to 5.25 Adaptor and a blanking plate to block off where the Floppy used to be.
Col
