Board Thread:Questions and Answers/@comment-32833373-20170811212356/@comment-28905095-20170813150550

Most likely the reason it doesn't work is related to you being to far away from the spawning area. And you need to hang around there for the mobs to spawn.

This is a major difference related to the Java edition, you (or other players) need to be close enough for the grinder to work. The optimal range you'll need to experience with, but 24 I think is a little high. On Java you do have some spawn chunks which makes having mob grinders somewhat easier (especially when you're not around).

In 1.2 you'll also get the `tickingarea` option which you can use to trigger the mob grinder to spawn even when you're not around.(That is if I've understood that option correctly)