log in to facebook key hash problem

Hello, In my game I figured out debug hash key in unity then paste it in facebook app page thensfter that I build and tried the game on mobile. In the first time I logged in facebook by clicking on "log in button" inside my game it worked perfect and everything was okey. but when I logged out and logged in again facebook showed me a message that key hash it's not correct and I havr to to configure key hash on Facebook app site . Really I dont know why this happens. Can anyone help me?