Man, there are so many variables here, but I agree with Pdvdh response as a starting point. Running a soft metal (brass) into a harder metal (steel) will always result in the softer metal getting formed (Resized). If your the base of your die is coming in full contact with the top of your shell holder and you have resistance or cam-over. That is as far as your brass is going to go into that die. Adjustments to shell holder or die will be necessary. Shell holders come in different thicknesses by manufacture. Taking a couple thousands off of you die is an option if you have a lathe or know a machinist.
Are you having issues with these cases loading into your chamber?
I ran into this issue in some 243 brass in the past a very long time ago. I was using an RCBS FL die and was not consistently bumping shoulder back on all cases. I started using a Redding Body Die and changed to Forrester NS Bench Rest dies and solved my issue. Body die only deals with case resizing and bumping back the shoulder. You can adjust the die to tailor headspace. I does not touch the neck of the case. Then size the neck with NS die. I typically only use body die every 4 or 5 reloading's. By this time it is time to anneal and you will need to full length size the case anyway or toss them. FL dies may not always bump back and can sometimes make matters worse?
The 243 by design with a 20 degree shoulder is another issue. I have found that since I have moved to cartridges using 30 and 40 degree shoulder that I don't have the case sizing and trimming issues.This may be getting too far down the Rabbit Hole?
How and where are you getting your measurements from? If you are using digital caliper, how accurate are your calipers? Are you using a comparator to measure base to datum?
I don't know if any of this helps to OP, but taking the advice of many on this forum and doing your own research is a good place to start. The first thing I would be to try a few different shell holders checking thickness. I know Lyman #2 will allow cases to go an additional .002 into the die. If that does not work, try is adding a body die into your reloading process. A few other pieces of advice I would have is adding a Lee de-capping die, I don't use de-caping pins in my sizing dies. I recap all of my cases after firing and clean before going into any of my sizing and loading processes. I have used Lee Collet Dies and like them for some cartridges. I like Lee factory crimp dies for my hunting reloads or where cartridge will see possible rough use. Here we go again, Down the Rabbit Hole! I could go on and on reloading for 50+ years of my life.
Welcome to Reloading, it is a lifetime of neck bumps and learning.
Once fired lapua brass
243 win
Rcbs full length sizer
Most cases bumping back as expected 2-4 thousandths.
6 or so shoulder is not moving at all, even as I crank the die down a lot. Removing expander does not change anything.
Example
Fired case base to shoulder is 1.658
95% of my cases are 1.6535-1.656 which is what I was shooting for
6 cases are 1.6580 and won't budge. I've turned the die almost to camming now and there is zero change.
Why are some of these being stubborn? What am I not thinking about here? Is it my press flexing on hard brass?
They chamber ok with resistance, so I plan to just shoot them aIf using a Hornady, are you using D400 or C375 bushing? You measurments tell me that you are using C375 and the correct bushing is D400. Base to datum should be 1.6240 on a new unfurled case. Measurement on my twice fired cases are 1.6320 and chamber effortlessly using D400 bushing. When I use
Once fired lapua brass
243 win
Rcbs full length sizer
Most cases bumping back as expected 2-4 thousandths.
6 or so shoulder is not moving at all, even as I crank the die down a lot. Removing expander does not change anything.
Example
Fired case base to shoulder is 1.658
95% of my cases are 1.6535-1.656 which is what I was shooting for
6 cases are 1.6580 and won't budge. I've turned the die almost to camming now and there is zero change.
Why are some of these being stubborn? What am I not thinking about here? Is it my press flexing on hard brass?
They chamber ok with resistance, so I plan to just shoot them as of now
I'm a bit confused by your measurements. A 243 Winchester headspace - shoulder @ .400. Using a D400 bushing in Hornady headspace comparator gauge, I get 1.6330 on a 10 case average for 2 time fired cases I have. This measurement is .007 less than maximum SAAMI specs for the cartridge 1.640 from bolt face. My rifle may have a tighter chamber and of course these gauges are not pin point accurate. The best use is taking measurements before and after case is fired. I am interested in knowing what measuring tools you are using.