My kayfun v4 has been working good for the past few months, but recently the screw at the bottom of the tank keeps coming out. So I took the bottom off, which just came off because it wasn't screwed in, and now I'm trying to put it back together but having issues.
Here's the two parts and the screw:
When I put them together, they don't fit properly... if I push part A in, then B raises and vice-versa. This means the two parts aren't close enough for the screw to go in and grip the thread inside.
In the picture below I've taken the ring off. This is how they fit together, C goes into D. Notice how those parts are threaded, this seems to be the problem. How do I screw those parts together when the fitting around it is square shaped? It seems the threading is keeping it from fitting together snugly.
Not sure what I'm doing wrong so any help would be appreciated!
Here's the two parts and the screw:

When I put them together, they don't fit properly... if I push part A in, then B raises and vice-versa. This means the two parts aren't close enough for the screw to go in and grip the thread inside.

In the picture below I've taken the ring off. This is how they fit together, C goes into D. Notice how those parts are threaded, this seems to be the problem. How do I screw those parts together when the fitting around it is square shaped? It seems the threading is keeping it from fitting together snugly.

Not sure what I'm doing wrong so any help would be appreciated!