It's tricky, with mine i just had to apple more pressure, ensure that its lined up. Also you might have to push the button that releases the back door to open up the hinge then push it in and lock it. Hope it works
Sounds like it's time for some duct tape! Kidding aside, bring it in to Tmo/Cingular whichever carrier you have and see if they can fix it or try a different back piece on it.