Sometimes it could be dust & lint trapped inside the USB port. Enough dust & lint had accumulated in my phone once, and the connector wouldn't fully insert itself into the port. Also, my USB cable would fall out a lot easier. It was hard to tell that this was the reason as to why my phone wouldn't charge since it looked like the connector went in far enough, but in actuality it was sticking out enough to prevent it from making good contact. I used a skinny safety pin to clean out the dust and lint inside the USB port, and that seemed to do the trick.