Carbide handgun dies especially, to my knowledge, only contact the exterior diameter of the case and have no mechanical means of making the case shorter.
They also most certainly do not reduce a case to its original dimensions; its usually smaller, more than what a steel die does, which is why many rounds made with them have a bulge. I'm assuming these are carbide dies, because that's what most use.
This smaller diameter size collar elongates the case; the expander run through shortens it.
I have to admit that I wasn't completely aware of this, so I went to my loading bench, got out my caliper and checked it out. That is exactly what I'm seeing. And the end product is the same length as the fired case.
I agree, if the sizer is not set up to size low enough on the case it can create the issue the OP is having. The case could be hanging on the pressure expanded ring on the fired brass that is not reduced sufficiently, deadening the firing pin blow before the extractor or headspace shoulder stops the brass.
I'm having a hard time believing that if the force of the entire slide mass isn't enough to seat the round thoroughly, the little firing pin is able to move it more. It's possible, I guess - but I've never seen that. I have experienced the bulge in a tight chamber stopping the slide out of battery, and cured that with the FCD.
