If you can't figure it out... Sell it to Gun Kid... sounds right down his line... I guessing that you have swapped Magazines... does the tip of the next round show any wear/damage from the BCG or is it pushed forward in the magazine??
I have swapped mags, checked to be sure the gas block is perfectly centered over the port, both heavier and lighter buffer springs, a variety of ammo types, including some hotter hand-loads, different bullet weights; it cycles with a suppressor.
Sounds more and more like it's time for an adjustable gas port. Suppressors generally slow the bullet however slightly, thus allowing a bit more time for pressure to build up, or at least be retained long enough to complete the cycle.
