Not sure what the configuration of your shower is, but here are some ideas that have worked before. If the screws were attached through the wall and into a stud...just get bigger screws. Might have to re-drill the shower frame. If they simply go through the wall into the hollow behind it...molly bolts might fix it. Or, the best way would be to drill new holes in the frame and re-position new screws into the wall. Thats a start of ideas anyway.