My Lands: Black Gem Hunting

can someone tell me how to get rid of the triangle the "!" in it, that shows above buildings?
i thought that if i upgraded the houses or built new ones or upgradedthe building, itself, then it would go away, but it doesn't. i know what it means, but i don't know what to do.
please help me. 0.0

It means you do not have workers working in the building.

It shows that you building is not working. There may be several reasons for that.

1. The building was damaged in an attack (of course only buildings in villages can get damaged)
2. There are no workers (not enough or they are just lazy and having great time at some party)
3. There is to much of the resource this building produces.