Mobs spawning everywhere except the mob farm. What can cause this?

Question

On my creative server (vanilla, i.e. no mods) Mobs are spawning everywhere except for the mob farm.

I have lit up all of the surrounding areas like caves and the surface and still no mobs. What can cause this? How can I fix it?

Answer 1

Just build a 96 block high pillar on the mob spawn area, and wait on top of that pillar. You should see mobs spawning.

Also keep the render distance more than 8.