Trading ports not trading?

So I'm wondering why some trade ports trade and others dont? For example, I have a highly developed trading port in Marseille, but it doesnt trade! And by trade I mean that it doesnt have that green line of trade from. I have trade notes occupied, but those grean lines only goes to my trading port i northern france. This is annoying since I have ports in many different places of Europe but they're not being "used". Is this a glitch, a problem or is this the way it's suppoed to be? Why are some trading ports used and others arent?

I've noticed this, too which is most frustrating since the AI tends to target these ports in particular. There is sort of an advantage to it though. Because instead of spreading the trade routes all over, and thinning your available protection, you can focus your fleet on defending the one designated port. I also think it's this way because the designated port is usually the one you start out with which makes sense because it's already a developed port ready for trading. Another thing to consider, is that all ports have a limited number of trade routes for capacity. So if you have more trade partners than is available for one port, the next port on the theoretical list (selected by random I'm assuming) begins the next set of routes until it's at full capacity.

Another theory I have is based on a campaign I played as Great Britain where my original starting port was next to London, but the designated one was at the Western most part of the Island in the Cornwall region. I believe that is because it is closest to the trade nodes which would explain a lot.

But, don't mistake having mostly trade ports as a mistake. Because a fully upgraded port, even one that isn't in use, generates more money than fishing villages (I think somewhere in the range of 400 gold a turn based on the region and it's wealth). So it still brings more money than any other alternative. So my personal advice would be to recognize the designated port from the start and keep it upgraded a step above the rest throughout the campaign due to the fact that the real value of an upgraded port is that it increases the value of the goods that come through it, in turn making more money for you in whatever you trade.

My personal question is how to, if possible at all, transfer these trade routes from one port to another based on convenience?

Destroy the port you don't want, trade will automatically shift to the next nearest one.

It's a kind of design flaw: trade goes through the closest available port.

Some recommand, when playing France, to destroy the port in Antwerp to force the trade through Toulon which is easier to defend.