The BLI's I have, M1, I1, H10, all the plugs/sockets fit more then snug, almost to the point it scares me to pull them apart and having the wires pulled out of the socket/plug. You should even be able to unhook the drawbar, and pull the loco down the track with your fingers and the tender will be pulled along by the wires. You can even pull the tender and the loco will follow along with out the plug/socket coming apart. I'd even hate to pull the loco one way and the tender the other way to separate the plug/socket.
Oh, here is a thought, look in the plug on the tender and count the holes (8), now look at the socket under the cab of the loco and count the pins (8), do you have a equal number of pins and holes? Maybe a pin is bent over?
About those ribs on each end of the tender's plug that match the slots on the loco's socket. The ribs and the slots as with the holes and pins are all set off center the long way, they can only go together correctly one way.
Now if those ribs are sheared off, I wonder if the plug could be forced in to the socket wrong one time and never work correctly after that? Look in to the loco's socket, the pins and slots should be in the lower half of the socket. Now look at the tender's plug, the holes and ribs should be in the lower half of the plug.
I'm going to bed now, it's 3:37am. here.
