Read somewhere that 5.45 mags need to have BOTH the follower changed and need some relieving of material on the underside of the feed lips to accomodate the shoulder on the 5.56 round which is farther forward than on the 5.45. Without such relief the rounds won't sit straight in the mag when it's time to feed.
Probably the reason that 5.56 mags don't work reliably as 5.45s, since the material is gone which supports the shape of the 5.45 case.