When I put the processed brass in the case gauge, many needed a little push to go all the way and don’t freely fall back out.
First thing that comes to mind is why some “resize” and case gauge and some do not. If the die didn’t perform its job, why do some work?
What you describe, I have seen often, almost in but something stops it right at the rim. If we look at sizing dies one will note that it can size the neck, shoulder and most of the body, except where the radius begins at the mouth of the die. The entire rim of the case is in untouched because that part is in the shell plate.
This is where most people get mystified when a firearm malfunctions and they “plunked” all of them in the barrel first. That’s because the barrels contact little if any part of the rim. A case gauge will catch imperfections caused by the extractor and/or ejector though, so a plunked round will pass but fail in a case gauge or sooner or later cause a malfunction going into the breech face or bolt.
You can push through or roll size the base to make sure it’s ok before you load.
or just take your rounds that almost make it and put them into the case gauge backwards and give them a little twist.
If that is not the problem, then I would want to runout check the ammunition to make sure it’s not crooked enough to fail.
I am reading that you are using the Dillon trim die as your only sizing die?
Using a universal decap die?
What expander are you using?
If you’re not using an expander at all what is the ID of a sized neck?
What case lube and application method are you using?