I've had this issue in Rome Emperor Edition for as long as I can remember, and I had hoped that it would be fixed in this patch. Unfortunately it hasn't been.
For whatever reason, the siege AI refuse to attack in multiplayer City Assaults. Singleplayer works fine.
Steps to reproduce:
1) Start a multiplayer lobby
2) Set up a 2v2 siege, with 2 ai on the attacking side and 2 players defending.
3) Begin the siege and wait for the enemy to never attack.
Here were my lobby settings:
Attached is my dxdiag.