Hi Tytr and welcome to the Zeus Heaven Forums.
Is that the episode where you need 11 trade partners?
When people won't trade with you, it's because you haven't opened diplomatic relations with them, or you've pissed them off, or it's been scripted that way.
The solutions are as varied.
1 You can try giving them gifts of things they need and hope they begin to like you enough to start trading with you.
2 You can conquer (sp?) them and make them a vassal where they not only trade with you, but pay tribute as well.
3 You can wait for a scripted event which will open the trade route.
In that episode, if i remember correctly, i had to conquer (here's that word again) one of the cities and the other opened from a scripted event. Sometimes you just have to wait it out.