I have a question on contibution in city sieges. I usually play a Deff Tank and most of the time I end up in the lower third or quater of contribution (16 to lower). Combined with unlucky rolls I end up most sieges with zero bags. Since I'm deff my dmg isn't great, but I do what a tank is supposed to do. I guard, I challenge, I assist on my dd target, I kick tanks, I stagger... but I still end up with apparently no contribution.
I do not have this issue with the same tank in Forts. There I usually end up in the top 20 and then get screwed on the dice roll (

So ... am I doing something wrong? Is this just incredible bad luck or is the system rigged against tanks?
What exactly gives you contribution in cities? Anyone got any tips?