Have you ever attempted to push the battery cable connector into the motherboard connection port but it does not fit correctly?

When this happens, there is a good chance that one of the pins inside the motherboard connection port are bent. Use one of the tweezers included in your ifixit toolkit to gently bend the bent pin back into place. See picture below. 

The red arrows indicate the holes on the battery cable which should align with the pins within the motherboard connection port.