If this is a duplicate I apologize, but my computer went off line as I submitted it, so I am not sure it went through.
Just a suggestion, you might want to try using expanding plastic or maybe metal wall anchors in the screw holes. Read http://www.naturalhandyman.com/iip/inffastener/infanchor/infanchor.html
about a quarter of the way down the page Plastic (and other)Expansion Anchors. I know that isn't their purpose, but they are cheap and it can't hurt to try.
Otherwise larger screws is the only other alternative I can think of - and eventually the same problem will probably recur.