Wait a sec, it looks like you don't have a fixed loop/locked brummel where it attaches to the hammock. That looks like an adjustable loop in section 1 (albeit a small one) and the thing sticking out is the tail that's been backspliced (which explains why you have a bead in a fixed loop which is really an adjustable loop).
I'm not saying this is why your whoopies are slipping - I'm just saying that should be a fixed loop, not adjustable. Here's Opie's locked brummel pictoral. This should be the first thing you do when making a whoopie sling, then the adjustable loop and bury.
The way you have it made, that adjustable loop on the hammock is bound to slip - the forces exerted on it are backward.