Anothe situation that leads to the 'do nothing' AI bug is river crossing battle:

I noticed this on my turk campaign, facing the Tims in the two river crossings north of edessa: If the AI has got artillery, then it will try to use it first, and send its troops only after 'something' happens. That something is either:

1. The art runs outta ammo
2. The art is destroyed
3. I launcha a full - scale attack

If i didnt have art on my side, none of the three things happened, and the AI would just stand there without doing anything... just like me, except that they were the attackers, meaning they had to... well... 'do something'