Hostile mob farm with flushing system not working

In my mcpe world I have built a 10 story hostile mob farm with an auto flushing system. The farm I used was mumbo jumbo's design. I afk 24 blocks from the center of the farm and I only get spawns for 5 minutes after loading the world, then they stop. From what I know mcpe and mcpc have the same spawning rules when I built the same farm on the pc edition I got incredible amounts of drops. Is there some bug that affects how mobs spawn in pocket edition?
PS: I have lit all nearby land and caves and have it in hard mode.