Hostile mob spawning in my mob grinder

I have plenty of worlds in Minecraft that have tall mob grinders. One thing that really makes me angry is when I waste several stacks of building blocks, several stacks of trapdoors, a bunch of iron, and ladders for my mob grinders to not have mobs spawning inside. There is no lighting, and I have no clue how I can help mobs to spawn. I think I am triggering spawns in caves, but I have an AFC (away from controller) area 100 blocks above the spawning platforms. PLEASE HELP! I don’t know what version I am on but it is the newest Xbox One update.