The kitchen lights in my condo went out recently, so I went and got a T12 electronic ballast to replace the old magnetic ballast in one of the fixtures (the ballast in the other fixture had previously been replaced). In order to do that I removed the fixture from the ceiling, and now I find that the holes where the mounting screws went are too big to accommodate them. I inserted a screwdriver in the holes to see if there was anything for the screws to screw into, but it just flailed around loosely and didn't hit anything. The ceiling is drywall, about 3/4 of an inch thick. I got some patching plaster, but I'm not sure if that will work. Any suggestions?