iPhone 4 battery not charging after screen replacement

I replaced the screen on my iPhone 4, the screen now works fine but not the battery won't charge.

When I plug in the charger, the icon changes to reflect this, however it won't charge at all. Just incase it was a dud screen, i put in my old one back in, still the same. Here's what I have already tried:-

• hold down home and power button until reset.
• reset over wifi
• reset through itunes
• tried a different battery

During the screen replacement i lost a couple of screws, would this make a difference?

Any help would be greatly appreciated, thank you.