Measure the Headspace before and after you resize the case. It should bump the shoulder back either .002" or .003" if it isn't your die isn't doing what it's supposed to. Also you can compare the headspace from a new unfired case against the one that doesn't chamber. I fireform everything and then bump the shoulders back .001" from the measurement I took from the once fired case evertie I reload.I don't know what sizer die you have but I recomend trying a Redding Die. If that isn't you problem sounds to me like a good like with a bore scope might be helpful.