Next to foreign objects in the sim holder, chances are that you or someone else used the wrong screw on the pictured hole. In my case, the holder didn’t close on the very last millimeter, which is why I opened it up again and checked for the reason. Took me a while to realize that it’s just this screw which was too long. Ha! But now it’s fixed, card holder snaps in like on a new phone. I am so happy about this. If in doubt, pls feel free to check out this useful manual @ ifixit http://www.ifixit.com/Device/iPhone_4