This is a shot in the dark but are you putting it on in the correct orientation? For example, with my Seidio I had a similar problem when I first tried to put the front piece of the case on. It wouldn’t snap on like it was supposed to, kind of like yours. I messed with it for 30mins before I realized there was a small cutout/hole on one side of the case for the mic that you talk into. I had the mic cutout on the top of the phone, the opposite of where it was supposed to be. Once I switched it around the case snapped on perfectly.
Like I said its probably not the problem, but just wanted put that out there just in case.