Sounds like one of a couple issues...
1 ) hold front of mag up while you cycle the rounds with mag full, if it works, the front mag tab needs to be tweaked to stay up in place
2) possible AOL of your dummy rounds is above spec, I think 30-6 is 3.34, measure and adjust them to 3.20 or 3.25 then retest (tight fit in mag can make top case base jump behind the base under it and then it tries to pull both out of mag)
3) too much spring pressure when loaded can cause too much resistance on top rounds in mag and cause many issues from spreading upper mag to causing cycling issues.
4) if mag sizes are all the same, 4 round, try one of the new mag springs in your old mag and see if problem is the same.
5) as always, alot of pictures helps, gun, broken spring, old mag, new mags, dummy rounds...we all like to look at pictures
6) The 760 and similar magizine rifles need mags to fit tight and perfect, just like it came from factory, learn to tweak them for proper lockup and spring tension, newer firearms with better tooling techniques has us buying multiple mags and not worring if they dont snap in perfect, they still work. The old rifles were made with one mag, a lot of times just a few rounds (hunting laws) and that was good enough. New mags had to be tweaked, just like a new firing pin or new ejector...