Could be headspacing, but also could be a slightly tight chamber.

My 12 LPR in 6.5mm was new when I got it.
It shot terrific out of the box.
Always showed pressure signs, regardless of factory or hand loads, even hand loads 25K psi under Pmax.
Turned out to be a tight chamber that Savage bored out a couple of thousandths.
I suspect that the original chamber reamer was at the end of its useful life and was worn.

Never had a pressure sign since the chamber was fixed and, 4,900 rounds later, the rifle still shoots as good as it did on day 1, and even better now that I know what powder and bullets it really likes.