This is just a wild guess, but since the mags are new, you may be experiencing a problem with the size of the base of the cartrage. Most companies will recommend that you use a X-Small Base full length resizing die. I use a RCBS one and no more FTFs. The other thing is to bump the shoulder of the case to the correct spec's. This is done by using a case length gauge when setting up your dies and press.
This is a case that needs to be bumped. Does make a difference.