International Trade Routes Broken?

I have noticed lately that some of my cities can start a caravan/cargo-ship route to another city but not vice versa.

I noticed this problem most recently in the early game when I had 1 cargo ship up and had a city across a lake from my capital, about 7 tiles. My capital could trade with the new city but the new city couldn't trade with my capital. Is there some sort of hidden requirement for trade routes ie X population or X structure built?

I planed to trade food to my capital to help make the use of production squares less starving.

Anyone else have this issue?

the source city of food trade route should have a granary built

Huh, thought it was the research that unlocked it and not a building. Fixed and thank you. They really need a more in depth explanation of International Trade Routes somewhere.