Why my miners are not using my stockpile?
Miners mine. Nothing more.
Laborers carry.
Simply said they don't feel responsible to move their stuff. Others will do that.
Laborer is a generic name in Banished for workers who do not have a set Profession. They carry out menial tasks such as clearing trees and carrying logs to storage as well as moving resources from one location to another.
On a wider note why the Laborers dont use that stockpile. Most likely they will fill them up in Build order or atleast try to fill the ones which are already in use.
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