I would inspect the female end of the battery that is giving the problems. Deans arn't known for problems, but it sounds like that connector has something stuck in the bottom of one of the terminals.
It's either going to be insulating the poles if for instance there is some film or plastic down one of the holes, or there is a lump of plastic right at the bottom from soldering that is pulling the poles to one side and it just can't bridge the small gap.
Getting a fine probe into the terminals and cleaning them out might fix your problem, Failing that solder a new female onto the battery (1 pole at a time for safety) may fix it.