I have the same problem and I have 3 trading depots fully upgraded for what I need. That are never full but yet factories keep asking for somewhere to ship the freights. --- Interestingly enough, video game companies used to be able to make money without selling out like cheap prostitutes.-KaiserWarrior
Have you turned on exporting for each of the things you want to ship?
How do you do this?
Click on your trade depot, and make sure you build the add-on lots for the particular commodity you want to export. Then click on the tooltip again and you'll see a "manage global" button, where you can toggle on exporting.