Without having seen your batteries, I can't be 100% sure, but I'm pretty certain you have batteries made of 2/3A sized cells. Those are the correct size, and there really aren't any shorter, but what I have seen happen is that the wiring between the first cell and the connector on the battery is not long enough.
The connector on the gun needs to be plugged into the battery and then fed up into the area below the rear sight on the AK. Sometimes the wiring on the battery or on the gun does not allow for this to happen.
A Batteries Plus location should be able to reconfigure your batteries as a tech build to have a longer amount of wiring between the battery and the connector.
I'm not sure, but Deans Plugs might also allieviate this problem because of their size.
Hope this helps.