New Battery wont charge from flat.

Hey all, i've just replaced a dock connector and battery in an Iphone 5, the battery came with roughly 50% charge. I charged it up to 100% and everything was fine. I let the battery drain, which was less than 12 hrs without use. Which was short. Anyway now the battery is flat, i've plugged it in and it won't charge. I contacted where i purchased the battery from and they replaced it. But again, i charged it up, let it run flat and again it won't charge. Could it be that i'm getting rubbish batteries? Could it be the dock connector, altho it charged both batteries while there was charge in them, would getting an exchange on that, cure it? Or could it be something else?

Thanks