I'm on a server using minimal plug-ins and found a zombie spawner. I've dug out a 9x3x9 space around the spawner, made sure the nearby caves are torched to prevent spawning, set the difficulty to normal, and made sure the light level around the spawner is 0. I do all this but the spawner still won't spawn anything.
I'm not sure what the problem could be and would appreciate any help.