A couple of things, are you taper crimping? Also, how much lube is on the bullets? I always do the plunk test when reloading lead. Fully complete a round, wipe off all the lube, and drop it in the barrel. Make sure you get the "clink".
I have had some boxes of bullets that had lube all over them. For these, I usually try to get most of it off before shooting. It can accumulate and build up in the chamber.