If was me having that problem; I’d know who to blame (me). Since Toby is better than a fair shot and has good equipment; let’s look at “could be”.
Powder charge such as weighing inconsistencies.
Neck tension. I lost a duel with a custom 6.5×55, because I had not learned the “voodoo” of case annealing. It dawned on me when I could not chamber a full length resized case; no matter how much I shoulder bumped it.
Barrel and throat condition. Probably not an issue with Toby’s rig.