I also have this problem on a converted world. All of a sudden the mob disappeared from the mob spawner, went to find another working mob spawner and created my new XP farm. Worked while I was in the game until I quit for a while, come back the next day and the same thing has happened to this mob spawner.
I also have this problem on a converted world. All of a sudden the mob disappeared from the mob spawner, went to find another working mob spawner and created my new XP farm. Worked while I was in the game until I quit for a while, come back the next day and the same thing has happened to this mob spawner.