Mob Grinder not spawning mobs

Oh you have AFK nearby so the mobs can spawn. (Very close to the spawner, like in the same chunk.)

The mob spawning mechanics aren't the same in Bedrock Edition compared to Java Edition. In Bedrock, the cap on the number of mobs is lower which basically means that there can't be as many mobs in your world. Bedrock also has a feature where hostile mobs (maybe not just the hostile ones I'm not sure) don't despawn. This means that the number of mobs won't go down in your world unless you manually remove the mobs that are elsewhere in your world. The problem with this is that you don't know where the new mobs will spawn and they will most likely spawn. Therefor, hostile mob farming in Bedrock Edition is very difficult and if you want a productive farm, switch to Java.