iPhone 3G lock button won't lock

I'm having some trouble with the lock button on my 3G. I press it and nothing happens. However, if I squeeze the front and back of the phone together (in the upper right corner) and press the lock button while doing so, it works. I've recently dropped the phone on that corner and replaced the screen, even though there was minimal damage to the glass itself. However, since then the button won't lock unless I squeeze the phone in that area while doing so. The feel of pressing the button itself seems no different than before. It's not like the button is out of place or anything. I'm guessing dropping the phone on that corner knocked something loose. Any ideas? Thanks!