I can't speak to the incubator but I have the old model "Crow's Nest" and even being a 3/4 length ... using a HG 20 degree Burrow ... I've slept really nice and toasty to the mid twenties.
The only thoughts I have that others have already stated would be it being snug to your hammock. I find myself needing to vent more than anything so I know how a little gap can shed heat rather quickly ... surprisingly quick too!
Wind can rob even the best quilts of heat so don't know if that's a factor in your particular situation.
The other point is everyone's personal "temperature rating" is different. If you're married ... you'll have first hand experience with this point!
I've woken up freezing before because my darling wife has removed my part of the covers for her own use!
The one cold butt cheek you mentioned makes me think you were either pulling it too tightly against the hammock or were bleeding off heat in a spot from a gap.