I had the same problem with my HammockGear Crowsnest. I did two things and have slept in it the past several nights with no slippage.
1. Offset the shockcord on the entrance side. Once you have the UQ installed under the hammock, take the shockcord that goes on the head ends and give it some extra slack to the entrance side where your shoulder will stick out more on an assymetrical lay. Then do the reverse on the foot end, giving some extra slack towards the footbox. This will make the quilt lay ever so assymetrical in line with how you'll be laying.
2. I then installed a "Te-wa mod" that he puts on all of his UQs. He adds a small piece of shock cord to the side where your shoulder will extend on the entrance side (where you just gave some slack). He attaches a mitten hook to a small piece of the shock cord and then attaches the mitten hook to the pullout on the entrance side of the WBBB. This keeps the shoulder end of the UQ permanently "attached" to the outside so that it won't slip out from under you.
You can see a YouTube video of it here
in which Te-Wa demonstrates how it works. It's 0:48 seconds into the video.
Doing both of these worked like a charm for me and has eliminated any issues I had.