My uncle passed away just before New Years and my father inherited his Thompson. We took it out and shot it a few weeks ago and the mag catch seemed to be sticky like it didn't want to lock the mags in place. It basically stays where ever you move it.  I suspect that either the spring is damaged or simply under powered, there is a burr somewhere causing it to hang up, or it needs to be oiled but I'm not expert.

I think I know how to field strip it but how do I get in there to check it out or swap out the spring if need be?

I'd start in the armory.

IIRC the mag catch on the new production Thompsons is slightly different. I believe you can get a surplus catch that will drop right in and work with old mags