Not that I do know but did you try to camp with one army for a few rounds?
Only thing I can imagine cuz it would be kinda stupid to force a horde into a settlement for growth...
On the other hand, horde mode (migration) is not supposed to be played all the time, I guess. Migrate from one place to another? I dunno. Playing Getes and rolling the map from top to bottom is my aim atm ^^
Oww, and did they raise the difficulty?
I am far from good in TW, actually I always sucked and mainly just enjoyed the battles, but it seems to be more difficult, even on Normal.