As to whether they will trade or not it can be a number of factors - firstly the 'balance' of the agreement - i.e. how much they will make out of it vs how much you will make out of it. You can see this by mousing over the trade route in question on the strategy map and it will give you a breakdown of how much each clan is earning from any given trade route per turn. If they do not go for an agreement sometimes a little bit of cash to sweeten the deal and offset any one-sided trade deals will make it work. As a general thing, the more trade goods you have the more money you will make from trade agreements therefore the less likely other clans will want to trade with you - a bit of a Catch 22!!

The other thing which does seem to be a factor is if the AI wants a certain trade good - a great exampe is the cotton from vanilla STW. Cotton is required for advancing the Yari dojo building chain, and the only source of cotton in the game is from a trade node. So if you control that trade node you have a monopoly. I don't have any concrete proof but my experience tells me that if the AI decides it wants cotton, it will pay an enormous amount of money to get it from you. I've sold trade agreements for a lump sum of 17,000 koku - which is crazy, but it can happen.

The final question about port availalbility - for a trade route to be established you must have a free trade port that is connected to your captial by land. This causes problems for Chosokabe who are stuck on an island with only a handful of tradeports and none of their tradeports overseas will ever work for them! Also, your propective trade partner must also have a free slot in atrade port - so often the reason you cannot trade is because your potential trade partner does not have the ports available.