It probably doesn't do anything either because 1) it's too close to the ground, 2) the game difficulty isn't on Normal/Hard, and/or 3) there are other players online using their own mob farms
I've made the exact same one on the server I play on, and it doesn't always work how it should
Another factor being that the mob spawning rate on the server is not set high enough, or if it's a server where the spawn chunks are always loaded, that's probably where the mobs are concentrated at