Could be a number of things
First off is if there is no connection by road to a market building. Second as you say, check your warehouse, if it is saying max on the iron etc. then yes you have run out of storage.
Also quick question, when a manufacturing or farm is full, and goods are stagnating in the building, are you still getting hit with maintenance?

Also quick question, when a manufacturing or farm is full, and goods are stagnating in the building, are you still getting hit with maintenance?