Slime Chunk not working

I have made a slime farm in a slime chunk and have lit up all 128 blocks around area. While I was working on the farm I saw slimes spawn. Once the farm was complete , it was working great I got 5 stacks in less than an 1hr. I went away and came back and it has not spawn one slime in over 3 hours. I have reload the game. I have went into peaceful and back into hard mode. I have went over 1000s blocks away and came back. I'm also standing 24 blocks away while afking. Does anybody know what I'm doing wrong? The only thing I did near the farm was breed some cows but I don't think that affects slimes spawning.