I had a similar problem when playing a GA game as the Danes (vi, unpatched). Finally gave it up. I've patched it since, and plan to go back if I still have a saved game.

I think that one mistake I was making was looking for my trade income by clicking on the port - it always said it was earning 0 florins. I learned later that to see the trade income I should click on the merchant instead.

Which makes me wonder - I've noticed that in trading provences I never seem to have any port income, but in provences that don't trade but that have a port I will earn a very small amoung of money (~10f) per turn. Certainly not worth building ports just for that, but it's the only time I see them earning anything. Is this what others find, too?

P.