How about a picture or two of what you're trying to do?
Is it a stock battery?
Could there be something in the battery compartment that shouldn't be there?
The wires should have room to fit above and on the left of the battery. Look around inside the shell to see where the plug will fit.
It looks just like mine does. You should feel a little pressure as you hear the "click" of the latch. The wires coming from the end of the battery will push against the door.
The battery should connect more securely, and should never come unplugged without some force. Your bird will fall from the sky if that happens in flight.
Putting the batteries in The P1 can be a fight at times. Plus all the cramming and twisting can eventually break them. Here's what I did and it's easy going.
First..tape the power leads over the end of the battery.
Second..slide the battery in with lead to the left.
Third....make a small notch on battery door for balance leads to stick out..I used a round file