Just to be sure, your freighter design have cargo handling facilities? Are they taking forever to load trade goods? Or just have no current orders/destination? I know freighters tend to go back and forth between colonies when they have nothing to do, with "Move to X trade location" and, once they arrive, a pause without order, followed with "Move to Y trade location".
This is precisely the reason why I don't give trade access to NPRs, then their freighters come in my systems and annoy the hell out of me with flickering contacts. At least you can filter that in your events screen...
I find that trade goods is an unreliable income source. In my current game, I get about 275k wealth per year in tax from population. In theory, I could get the same amount in taxes on export+shipping trade goods, if all trade goods were used properly. But since supply/demand of the various goods isn't balanced, I get about 53k wealth at most (20%).
I assume you've made sure the supply contract can be fulfilled (said installation present in the colony), demand contract is in a system accessible by jump gates in 4 jumps at most, and all those systems are safe (no red numbers in the galaxy map). Civvies are usually most diligent to fulfill contracts, and will prefer those to trade goods.
Hope I helped somewhat.