what's wrong with my battery?

[notnodak]

I have a 2s2p 5200 Flightmaxx battery that will not connect to my esc. It charges great and I've even cycled it on my charger. Everything works good till I plug it into my mamba in my E-Maxx.
Here's my readings.
8.39 at the main power plug
4.19 on one side of balance plug
4.20 on the other side of balance
8.2?when plugged into one side or the other of the esc plugs

why the power not flowing through the esc if I can read power on the other plug? all the other batteries do. I even tried plugging in two known good packs and all is well. when I remove one and plug in the problem battery I get nothing.