Help - Local Auto Trading Fleet isn't really doing it's job

Hello all,

This is my first play through, and I've turned Heretics End into a hub of industry, with a gas refinery, ore refinery, hull parts factory, clayronics factory and another one that's making everything I'll need for building ships (although I haven't gotten that far yet).

Now I had this genius idea that rather than setting up each station to have its own traders for sending goods between my own stations, I thought I could set up a trading fleet that would look for local trades within 1 system jump and set to only look for trades to and from my own faction.
This fleet would be in addition to each of the stations' own traders, and I thought, would mean none of my stations would be struggling for goods..... I was wrong.

I just noticed that a couple of stations were running really low on some requirements, and production had halted.
Then, when I checked the neighbouring station, it was full of what I needed, and there was a small shipment planned between the 2 stations.

But this job wasn't being carried out by my system-trading fleet, and when I checked them, they were sitting around doing nothing and had been for a while.
They were sitting there with 5% of their cargo full of something random, either saying 'No valid trades found within sector (1 hour ago), or 'not enough cargo space to complete job (1 hour ago)'

2 of my 5 traders in the trading fleet seem to be doing their job, so the config seems ok. But the other 3 are struggling.

Any advice or recommendations? Has anyone else tried this or found something better?
Surely I don't just spam all my stations with traders and hope for the best?