I don't think that the problem is with the Armalite lower, since the original mag supplied works just well.
I suspect that the problems lie with the IWI antiquated follower design and very stiff spring. Initially, I suspected the incorrect pinning; but even loading 4 rounds instead of 5, I was getting loading issues.
Keeping in mind that the problem is fortunately intermittent. It may go away with the mags breaking in. I will also try later to disassemble and polish the lips.