I have noticed that id a ship docks at an exchange then it is the only source of items that shows your inventory, so if you had, say 30 Iron on a second ship that docked after that at the same exchange - then try and sell it - you show as having none unless you move it over to the first ship that docked!
Storage Location will have a list of your storages at that CX, your two ships will show there.
Just select the 2nd ship and the Inventory Number will update.