Your battery is catching on the underside of the ridge that the big dropin spring sits on. Load the battery from the head, it will then clear this ridge. If you want to see what is happening, load the battey like normal from the tailcap. Replace tailcap and when it starts to get tight, stop. Now remove the head and you will see how the battery is jammed up.
Another way to do it is to put the head on the empty tube just so the threads catch and hold. Load battery from tail and make sure it gets in the hole and is on the spring. Bounces when pushed. Then screw on tailcap all the way. Now the head can be tightened all the way.