7mm rem. mag. is my pet cartridge so I'm going to see if I can help you here.
1. Are the cases that are separating factory rounds?
2. What powder and charge are you loading with?
3. With said bullet and powder charge what is your COAL ?
4. What do your primers look like?
5. How many times has the brass been loaded?
6. What bullet and weight are you using?
7. What primer are you using, brand, and magnum or standard?
8. What sizing die are you using, FL, or neck die?
The reason I'm asking these questions is, belted magnums headspace at the shoulder after the initial firing. Headspace is of little importance if your hand loading, and especially if just necking the brass. I load absolute maximum listed charges in both of my 7mm mags. and have shot a bunch through both and never had head separation, except when the brass has expired after 12 or cycles and then it will only show mild signs, but never problems with extraction being dificult. So, I would venture to guess if the brass has not reached it's number of cycles limit, you may be dealing with a powder weight problem, or seating depth driving pressures way up.
Review my questions and that of the rest of us, and let us know. Head separation is a very serious matter.