Just like people, cargo has a set destination. Upon creation of that unit of cargo, its destination is determined all the way, including it as a delivered good. You need to convince “the factory” to ship cargo to the destination your line is going to. If the steel mill you want to ship to has no output, or no set destination for its good, your cargo bay will not be used and by-passed. The entire route from coal+iron mine, via steel mill and goods, till a demanding city has to be available. Only then it will ship. When a city has reached its goods limit, the city will not demand, and good will flow somewhere else.
Probably somewhere in your chain there is an interruption, which sets the demand for coal+iron via your line to zero = no shipment.
At least, this is what I have experienced during my game plays.