Need help with mob farm

Hey everyone first time poster so I’m not sure if I’m in the right spot but I’m playing on a bedrock one block map and I’ve tried to build a mob farm. I’ve done everything between making sure the light isn’t an issue to making 4 layers and spider proofing but it’s still a super slow spawn rate. There is no other places they can spawn within a 120 block radius and nothing has helped. If you have any advice please let me know