How to make Rebel Armies attack cities?

[Atilius]

In LotR TW (unofficial campaign) they've attacked my cities.

The problem is not they won't ever attack, it's that they only rarely attack.

I thinks it involves the size of the guarison in the city and the troops in the army.

In our case, the settlement in question contains only two units and has no walls. The rebel army is a full stack.