Check your brass for donuts inside the case neck at the junction where the neck meets the shoulder. Possible that repeated sizing has caused a ring of brass to form inside the case which is putting up significant resistance for the mandrel and causing the case to collapse. The way to check for this would be to take a fired, unsized case and see if you can press or drop a bullet into the case without any resistance. If it drops in freely then I doubt donuts are the issue, if it hits a hard stop partway in then you've probably got donuts.
For an expander mandrel I would prefer to open the case up closer to 1 or 2 thousandths from FL sizing, but 0.004 isn't out of the question. You can adjust this with either bushing size or having a die custom honed to the desired neck diameter.
I suspect you are using an "expander" mandrel rather than a "turning" mandrel. They are 0.001 thou different in size, with the expander mandrel being the larger of the two and typically 0.001 under bullet diameter. The turning mandrel is 0.002 under bullet diameter. I use the "turning" mandrels as I prefer 0.002 of neck tension. Switching to a turning mandrel will reduce the sizing force needed.
Upgrading to a better mandrel material will also noticeable reduce the amount of friction/effort for sizing with the mandrel. Look at either the Sinclair Carbide, 21st Century TiN coated, or the 21st Century Black Nitride turning mandrels.
