I've made it to the point where I have a couple of colonies a few times so far, and only in iron mode. However, every time I do, eventually, these colonies' exports stop making income, seemingly inexplicably. The only things that maybe give a clue is that pirate attacks start happening around the same time, and that on the global market info screen thingy it says that if you're making no income, it may be because it's underground, which would make no sense considering it's regular goods like ore or volatiles. Something anyone answering might want to keep in mind is that I've stopped having free ports, because of the constant merciless attacks on my colonies by the hegemony and the church. But yeah, pirate attacks still happen, and, as I said, around the same time, but I couldn't find any connections between them and the loss of income.