I am currently playing on a server and found a place with 2 skeleton spawners close to each other. I created an XP farm but every once in a while, one of the spawners simply does not spawn mobs. The mob inside just spins faster and faster then slows down without any mobs spawning. So far, when this has been notice, the only way to fix it was to restart the whole server. To get to the spawners, I have a nether portal next to the spawners and in their "activation" range. This could possibly have something to do with it?