What is it about t-nuts that I don't understand?

I'm doing a build and usinf t-nuts for the first time because I
want a removable back and front for not only interior adjustments but
also possible replacement panels with a port cut in.

I have these things from Parts Express called Hurricane nuts.

The goal was to have these things in pairs on the frame and on the
removable back.

I got my corrent jole size drilled through both pieces and when I
got the t-nuts inserted and tried to screm them down they backed up
and loosened a joint. The joint is a small problem to fix and I'm working on the back to catch any of these mistakes before doing
the front.

In playing with the bolt and the t-nut, there seemed to be a stop
at the end of the nut in the machining.

I want the t-nut on the surface of the removable panel to use as
a magnet catch for a grille on the front side and a mated nut on the
frame to draw down around a seal on the frame that I'll fabricate.

This means that I was figuring on one bolt going through the first and the second drawing down tight to the frame.

Where has the logic or selection of parts gone wrong here?