OP I have the same problem as you, I cannot get one of my sides to click in. I just got my phone a second ago, and figured I wouldn't have any troubles with it.
After examining the one side that wouldn't click in with the battery door off, I noticed it is actually sticking up compared to the rest of the phone. I am almost positive this is my problem.
Does anyone else's look like that? Where it sticks up on one side, mine is on the right in I am holding the phone with the battery to me and the screen down.
Is this the same problem you are having? If I bought this over the phone and already used my one time return will they still give me a different one? I assume so, but one store I go to always gives me trouble.
Also could I take this into the store or does it have to be shipped out again?