Take a close look at your batteries. There are two small catches and one may be broken or missing. If this is the case see if you can get a replacement set from GoPro. These catches are removeable and can be swapped with a small socket-driver.
Right, check your battery that works and you will see a little ramp that allows the battery to lock in place on either side of the battery.
Now on the battery that does not work I suspect that one of these ramps are broken and preventing the battery from locking into place. I have added this photo. You see the ramp where my finger is placed, there is one on either side of the battery. This is where I suspect the damage is located.
I had the same issue, called go pro sent pictures got a.new battery, then tried the battery in question and with some side to side wiggling when installing the battery engaged and locked still to this day gets hung up some days and other no problem, believe me I studied every inch and their identical..keep trying