I had a similar issue with traders when I installed this mod. I saw a CJ that had traders on a lake attached to a tributary, and tried it myself. Before installing this mod, I had traders on small tributaries and lakes connected to them, no problem with attracting traders. I started a new map with this mod installed, now the traders do not visit the posts up the tributary, only the posts on the major river and lake through which it runs. The only difference with the new map so that I have your bridges on the tributaries at some point, it seems it doesn't matter if the post is before or after the bridge, the trader doesn't show, he/she just paddles up and down the river but will still visit the posts just around the bend on the main river. Not sure if it is an issue with the new map for some reason or if it is with the mod or a combination of the two but it sounds similar to the previous post so I thought I'd share.
I don't have screen shots atm, nor can I download the update to test yet because my rural internet here in Nova Scotia is so terribly poor I will be surprised if I can post this message!
Update - I'm on another map now, same issue.