I think I have your answer as I have the Hammock Gear 0 degree under quilt and had the exact same problem. What's happening is there's a little air bubble gap between the bottom center of the quilt and where your butt is in the hammock. You don't see it because the "ends" look good and buttoned up. A way to check to see if this is the problem is to "sit" in the hammock with both legs over the edge like you're cooking or putting on your shoes; then reach under your butt and tap the quilt to feel if it's tight up against the hammock and your bottom or whether there's a small pocket of air. If there is, that's your culprit.
There's two shock cords on the suspension of HG UQ's. the first is a heavy duty loop; when you clip the S-Biner to one end and the other to the other end one this is the main suspension. On this one you can slide the quilt left or right freely. Then there's another shock cord at each end. This one has adjustable length and it too clips to the end of the hammock useing the same S-Biner, the sole purpose of this one is to "adjust" distance from the end of the hammock to the start of the quilt. keep in mind there's one on each end. if these are too long or loose, then the quilt sags at the very middle when you sit in the hammock and that little pocket is created right below your butt.
If you have the gap, you simply shorten those two shock cords on either end and it'll "stretch" the quilt lengthwise a bit more. You don't want to do it too much but enough so that when you "sit" in the hammock you can tell that the quilt is pulled tight to the bottom of the hammock.
then there's a tiny shock cord that just gather's the quilt at each end to keep wind out; you can crank those down but that won't have any effect on the "butt gap" issue.
I had the same problem and was warm but not like I felt I should be with a zero degree full lenght underquilt. Once I figured this out and made the "slight" adjustment it was like someone suddenly plugged in an electric blanket on high. I figured this out 2 nights into a 3 day trip. I couldn't hardly sleep that night because I was so hot and had to vent on the top.
I hope this helps; I think it's exactly what the problem is. Again, you can feel this by sitting in the hammock and reaching up underneath and patting the quilt under your butt; you can tell when you do if there's an air gap there. Any air gap will significantly reduce the effectiveness of the quilt.
Enjoy the day