Just curious, but what methods are you guys using to try to take it back? The last word we've gotten is that the best way to reclaim a beastman controlled region is simple slaughter. Anything works in that case, even Too Weak. It's a bit diff when competing w/ the other nations, but running around with random Subduction spam seems like it should work faster than what you are seeing. :/
Of course, that info is a bit older (Feb 2016), and they've been talking about conquest issues on JPside for a couple months (and supposedly working on some sort of solution to at least counter the effect of BCNM losses,) so it's entirely possible that the kill values are just too low still and unpopulated/unpopular regions simply can't provide enough targets for a small group to retake them. >.>;;