Rear panel not fitting after repair - iPhone 4

Hi

I have just replaced the home button and ear piece on my iphone 4. Put everything back together quite easily but the rear panel wont slide on anymore, the battery appears to be higher than the frame. I have managed to force the rear panel only so I can use the phone but its not secured in the top right corner near the vibrator.

What could be the issue? There is nothing obstructing the battery, everything is in place perfectly and working correctly. Could I have done something to the battery to make it bulge? I made sure the wifi antenna is correctly in place so that doesn't appear to be obstructing the panel, it just appears to be the battery.

Any help would be much appreciated.