It's unlikely that you're fast cycling and failing to lock back unless your mag follower is really slow, so I doubt more mass would be the answer. Where and how far are your empties ejecting?
To go lighter, you'd have to go gamer with a TACCOM buffer or take weights out and replace with nylon or delrin rod sections. Not sure what your intended use is, and how comfortable you'd be with either of those solutions.
If it is indeed leakage, it usually gets better with carboning up, but YMMV.

