I'm in the Timna mission, have a granary accepting chickpeas on the kingdom road, have a trade route to Thinis, am importing to maintain 2000 chickpeas, but the traders walk through saying my city won't trade. I had no trouble with importing bricks in the tutorial; I can't think of what I might be doing differently in Timna. Any ideas why trading doesn't happen?

I am also exporting copper, and have a storage yard with copper on the kingdom road, but it won't pick that up either.

Part 1 of an answer is that I goofed on buying food: I have to have a storage yard accept the chickpeas, not a granary. Part 2 is still open: why, when I had copper in a storage yard on the kingdom road, did the trader not buy any of it? It started buying after I make space available for it to sell me chickpeas, but I don't see what that has to do with the copper.

Conditions to trade are:
1. If the city buying from you is trading via land you need: trade route paid, storage yard with that product connected to kingdom road, the product in question marked as "importing" in the commerce overseer (and not stockpiling).
2. If the city buying from you is trading via water you also need a working dock, connected via road to storage containing that product, and the dock needs to be able to trade the product (in the dock properties)

LE: just checked and in the Timna mission you export copper by land; since you described you scenario pretty well, you should check that you do not stockpile copper;
also, if you are producing weapons, you should also check that weaponsmiths carriers are not on route to pickup the copper in that storage, since I think that will prevent the trader from picking it up too

Thanks! Your last sentence was especially enlightening since I hadn't thought about workers bringing copper *back* to the weaponsmiths near the copper mines. In any case, it might have been coincidence.