The only way to know if you are bumping the shoulder back when FL sizing is the Hornady LNL gage with a correct sized headspace bushing. Check this dimension of a fired case. Then properly lube and FL size a case. Measure again. The sized case should be .001 - .004" shorter. Now, here is the trick. . .different amounts of case lube will affect the FL sizing die's effectiveness. That meansfrom almost dry to just a bit over lubed will rresult in about .008" variation in my experience, so applyan amount of lube such that all thecases size with the same handle force to control this. I use Hornady One Shot, but there are others.

Also,if the FL die is nottouching the shoulderat all, the case can grow in length. I would guess this is what is causing your issue. Use a headspace tool as suggested and you will be able to see and adjust what is happening.

If it is not that, I would get a micrometer and start checking diameters between fired(from your gun)and sized brass. A problem here will most likely leave you sending the die back to the mfgr to have it replaced. i've never seen this, but it is possible.

Also, what action type is this? I was assuming bolt action. It makes a difference.