New fuel sending unit installed, plug doesnt stay on good?

[WH23G3G]

I just put in another new fuel sending unit because I broke the stud on the other one where the plug plugs in. It didn't seem like it was very secure on the other new one, so I thought it needed to be pushed harder and the plastic broke and the stud fell out. I hope nothing fell inside my new tank. Now the new sending unit is the same way. It has a stud screw where the plug plugs in but it doesn't stay on good at all. And I don't want to push it too hard again. It's a screw stud with threads and I don't see how the plug would stay on there anyway. Is this a different design than the original? What else I can do to make sure the plug stays on good? The way it is now, it would definitely pop off driving down a normal road.