What type of dies are you using? FL with expander, FL with a bushing and expander or FL with bushing only. Or a Neck die with any of the above?
I can't see how shallow seating provides enough tension and deeper seating doesn't provide enough tension. I don't think you are sizing your necks enough, if you measure a cases neck after sizing and compare it to a case with a bullet in it, what is the difference between the 2? This difference is the amount of neck tension, or interference fit.
I run .002" of interference fit in all my hunting rifles, and .0015" in my long range rigs due to them being single fed.