Sounds like your case necks are being closed too tight, so bullet sits on top of case and instead of going in, just crushes the neck. Some suggestions:
1. Try putting a tiny film of case lube just inside the case neck. You can try just rubbing over the top of the case with a tiny bit of case lube on your finger. I've used the case neck cleaning brush that came with RCBS case lube kit. I just take the brush and lightly roll it over a barely lubed lube pad like a case. Wipe off excess from brush and then put brush into the case mouth and remove it prior to adding powder. Some powder might stick to the lube, but I've never had a problem. You could put the lube in after powder load, but I don't. I've heard that Hornady One Shot Case Lube in the neck works as it is a dry lube so doesn't contaminate powder, but I've not used it.
2. Your expander may be too small in diameter, allowing the neck to be sized too small. Get another one to see if it works. Send email to Lee: they might even send you another one free.
3. After you sized the military-crimped cases, was the decapping pin assembly rod sticking out of the top of the sizer die? It should be flush (or barely above) the collet. This MIGHT have moved the expander ball too far up to work. Not likely, but worth a try. One annoying thing about LEE sizer dies is that the collet holding the decapping pin, expander rod assembly has to be REALLY, REALLY knucklebusting tight to keep it from moving up in the die when it encounters resistance, like removing military crimped primers. Lee told me that the decapping assembly rod sticking up 'a little' was no problem.
Good luck. Rifle cases are different process than straightwall handgun. Hang in there