MC Launcher keeps logging me of

So when I enter MC Launcher the login screen is popping up even when I am logged in and I ticked the don't log me off box. When I am in the game and I entered a multiplayer server and I leave this server and I want to join into another this screen is showing. I am using iMac. Sorry for English but I am from different country. Please what could I do, it is so frustrating.