Slitherine

I am playing successfully as Etrusker and have Venetia surrounded from all sides and the Sea.
When I attack the Boii in Venetia (there only province) I can not even harm any of the defenders. Even when I attack from all sides with the maximum of troops the surrounding provinces can support and additionally from the Sea. My attacking armies are hand selected for swamp warfare but I can not even harm any of the defenders. I play on difficulty easy! I am almost the biggest empire on the map and can not subdue the tiny tribe of Boii with any effort or trick? What is wrong here?

I'd need to see specific details of the region and armies to comment.

If you attack when the swamp is frozen with lots of Italian Foot (Medium Infantry) and skirmishers rather than heavy infantry and cavalry then your forces should acquit themselves well. Keep at it, if you use multiple armies then stagger them so they arrive separately from an adjacent region and from a region one space further away.