Strange things happening at a siege

[Austen]

In my present SP game, I am trying to wipe out C'Tis and am having two strange things happening.

First is that their 1400 unit stack keeps disappearing and reappearing. I have their capital of C'Tis completely surrounded with occupied territories. Twice now, I have gone to attack C'Tis with it's 1400 units, and then the turn I attack, I am sieging the fort, but their was no battle and they have no units left in the fort as far as I can tell. Then a couple turns later, the stack will suddenly reappear again. Dunno what is up. Are they hiding in the fort and I can't tell they are there?

Second, an dmore problematically. I have 300+ Units their sieging. Lots of big units too. 60+ Vine Ogres, heavy infantry, crushers, etc. and I cannot harm the walls. I have even equipped 5 leaders with Gatecrushers. Still nothing. my 300 guys + Gatecrushers are completely powerless against this fort. Never had this prob. before.

What am I missing?

TIA