You've got the bulkhead inserted from the outside towards the inside. Reverse it.
Put the rubber gasket on the bulkhead, nice and snug to the flange.
Then insert the bulkhead into the hole from the inside of the tank, so the nut screws onto the bulkhead outside the tank.
Hand tighten.
The seal was probably messed up from getting jarred around in the move. It really doesn't matter which way it is, this isn't magic, you probably just had a better seal when you put it back in the second time. I can show you entire stores with the gasket and flange on the outside so the systems can be closer together to save space and never leak. Or maybe the flange can only be on the outside in the southern hemisphere... kind of like how toilets swirl in reverse?????