It’s possible that it is a player error as
@rangedhealer77 mentioned.
If you’re aware of them not spawning for anyone else on the Skyblock server, it’s very possible that it’s simply an issue with the server, and could be looked into or rebooted to see if that helps - sometimes spawners stop working if TPS drops, or the server gets quite laggy, which can usually be resolved with a reboot.
If the issue persists even after checking your setup for the grinder and ensuring you have enough light etc, I’d recommend speaking to a member of staff incase it is a server-side issue.