I have had the same problem. I have used ultra lite drywall spackle and fiberglass drywall tape. Take the tape and place it down over the weak sheeting, it is self adhesive, then spread the spackle on and build up the weak area, let it dry and sand it down. It will give considerable extra strength. I also use the technique to build up areas that I have over sanded when I shapped them.

The Fiberglass tape gives the spackle something to stick to,and when the spackle drys on it, it will be more solid with the tape.

Give it a try.