Because i have measured the case before and after firing it?
You said, "I'm wanting to set my die for headspace but brass is not wanting to stretch in the chamber ? "
I interpreted "... set my die ..." to mean that you want to set back the shoulder so that it is .001 or .002 shorter than shoulder length - the standard guideline for shoulder setback.
I interpreted "... brass not wanting to stretch ..." to mean that you take a sized case, measure the shoulder datum to base length and observer X. You shoot that piece of brass and you repeat the measurement with the fired case and it is still X.
I'm not sure I understand the "wanting" part. In my limited experience, cartridge brass has really limited motivation to do anything. You have to hit it with 55,000 PSI at high temperature to get it to even pay attention. Some men and women have reported ex-spouses with similar motivation.
If your brass is chamber length (that is X is chamber length) and your die is not setting the shoulder back, this would be the situation. The brass is X, X is also chamber length, when you fire it it is already chamber length so it doesn't get any longer- voila, no change. If your brass is less than chamber length and you are shooting with low chamber pressure, this might also happen - insufficient pressure to expand the brass. There are probably other possibilities, those are the ones that come to mind right now.
Take a piece of fired-but-unsized brass. Measure shoulder datum to base. Adjust your sizing die so that when you size the brass, the shoulder comes out a few thousands shorter - I think that someone above said .005. Now shoot it. If that case is now .005 longer then adjust your die from there. If it is still .005 shorter than the fired case, I think pressure is not high enough to cause the brass to stretch.
There is an implicit assumption that your tools and your technique are correctly measuring the distance from shoulder datum to the base. I use the RCBS precision mic. When I look into the micrometer thimble I see a large hole and a small hole in the middle of it. When you tighten the thimble down on a case, the case neck goes into the small hole. The diameter of the small hole is .400, the diameter of the SAAMI datum line and is way bigger than the neck. The measurement is the distance from that "ring" (where the case neck goes into small hole) to the bottom of the micrometer. When I am working with this problem, I tumble the brass to make sure it is clean before I measure the fired case length. I have two 308 bolt guns, one chamber is .002 shorter than the other, this means that a cartridge that is right for the short chamber is a couple thousands too short for the longer chamber.
When evaluating whether the brass is actually chamber length, I remove the firing pin and the ejector pin from the bolt. I have extra roll pins. The bolt to case fit is still slightly constrained by the extractor. If I remove that, I have to poke the case out with a cleaning rod so I test a few times to be sure that I am getting the correct feel. If you think that your brass is shorter than chamber length, stick s small piece of scotch tape to the bottom of the case. If the bolt closes on that then the case is at least .003 shorter than the chamber.