All good info so far.
To eliminate possibilities, you now need to take measurements.
First off, measure these points on a NEW piece of brass.
.200" up from the base, you can use you caliper to scribe a line there.
The neck diameter, without a bullet, and with a bullet.
The shoulder position with a set of headspace comparators.
With a fired case, measure all of these again.
Regarding neck clearance in the chamber, it should be a MINIMUM of .003" LARGER than a neck with a bullet in it. My personal max is .005", but I see many at .006".
The measurement at .200" up from the base is critical, it should not be more than .005". And, it should measure the same ALL THE WAY AROUND THE CASE DIAMETER. If it changes, your chamber is an egg shape. I use a micrometer accurate to .0001" for measuring case diameter, a caliper is not accurate enough.
Check how a fired case feels when you chamber it after firing when the case is cold. If there is resistance, then your chamber may not be round. In that case, use a sharpie and colour in the entire case and carefully chamber it and carefully remove it, I normally do this with the firing pin assembly removed and the plunger ejector, if one is fitted.
Look to see if only one side or the ink rubs off.
Give all this a go, and get back to us with before and after measurements, the answer lies there somewhere if it truly isn't a pressure issue.
Cheers.