1. If you occupied more than 2 trade nodes, chance is, they don't want to trade with you. Temporarily moved your trade ship outside of the trade nodes and ask trade agreement again.
2. Get Horse Resource.
The A.I is pretty much coded to LOVE Horse resource. When u acquired horse resource, you can ask trade agreement to pretty much everybody and they ALL will accepted. they love horse so much i once offered a trade agreement and demanded some major clan (forgot the name) for more than 18000 koku in my Oda campaign and they were still happily accepted!
In another words, horse is the best resource, even better than gold!