Mob grinder

Are you playing on Survival, Economy, or Classic?
Some common things to watch out for with mob grinders:

1. Make sure the light level is dark enough inside of the grinder. Walk around inside with your F3 menu up, and make sure most of the area is at a light level of 7 or lower. Mobs will not spawn in areas with a light level of 8 or higher.
2. Place torches on the roof of your grinder chamber so that mobs don't spawn on top of the grinder. I frequently see mobs spawning on top of grinders instead of inside of them, causing the grinder not to work.
3. Try to stay about 24-32 blocks away from the grinder chamber. Mobs won't naturally spawn if you're standing closer than 24 blocks to where you want them to spawn, and they won't walk around if you're further than 32 blocks away. I like to kinda walk around getting closer and further from the chamber so that I don't have to worry about measuring exact distances, while also encouraging mobs do spawn in different areas and hopefully move around.

Let me know if any of that helps and if you have any more issues