first of all I'd like to add that I mostly experienced such behaviour on the lower strat-map settings, playing on very hard makes the AI generally a lot more fun ... and not just cause they are a bit tougher, but cause they actually do stuff (like attacking when sieging instead of just standing there ...)

anyway afaik all you can do is change the .. er .. whatsitcalled .. that "economic/agression setting", the thing you set in descr_start (like; religious caesar) and see how factions behave with diff. settings (this "can" have a mayor impact on how the AI plays!!)

or *somehow* script in to have the AI follow up on sieges ..
(and I'm not even sure that might be a posability in this game)


G