So I have created a mob spawning room and am standing 32 blocks away from it, its completely dark but no mobs are spawning in it. I am on the Direwolf20 pack for 1.12. the floor is all vector plates leading to a hole in the center where the mobs drop onto diamond spikes.

I do know that if it was cursed earth beneath the plates the cursed earth mobs do spawn but standard spawns are not working,

I take it you solved the issue? :p