Board Thread:Questions and Answers/@comment-32833373-20170811212356/@comment-28272355-20170813044016

Hmm... judging by that, I don't think there's anything wrong in that. If the floor for the mob grinder is entirely made of stone slabs, especially the mob spawners, that would interrupt the spawning process.

I think you should refer to Gamepedia for further details of this.

https://minecraft.gamepedia.com/Tutorials/Mob_grinder#Minecraft_Pocket_Edition_Mob_Grinders