Alright, well, I've narrowed this down to yes, in fact, the skeleton I'm using doesn't support belly growth, after having issues with SexLab Procreation increasing breast size but not belly size.
Why this is, I can't figure. My installation method was: UNP --> UNP BBP --> XP32's Skeleton --> XP32's Mega Pack (animations) --> b3li's UNP BBP Pregnant Model.
If it matters, I'm also using Children of the Sky paired with the ECE to make a late teenish character (not a prepubescent). The issue occurs even with other races, however.
I am unsure about those particular mod conflicts, but I would try a different order of installation:
Try uninstalling the XP32 Skeleton and reinstalling it (be sure it's the latest one by the way), so it is in the stack last as the later mods could have overwritten the "skeleton.nif" that has the "NPC Belly" node in it. Then, to make sure, run "GenerateFNISforUsers" just in case (probably not necessary, but since you have animations, I assume you might need to for any change to animations/skeletons).
Also, make sure you have a body that can accept changes from the "NPC Belly" node--b3lisario's UNPB BBP Pregnant Body (beta) is an UNP BBP body available on the download page. So far, since there are no clothes/armor that have weights assigned to the belly node, you'll only be able to ever see the changes while the character is using the base mesh (naked). Setting your options in the Estrus Chaurus menu to the highest numbers/settings should show obvious growth as in-game time progresses (every 24 hours for the 30-day period).
Hope this helps!