4 comments
when you go up the bubble elevator to the top, they start spawning, but not at the bottom
Spawners require a player to be within 16 blocks. You should notice some flame particles inside the spawner "cage" if you are within range. Funnel the zombies into a soul sand elevator before dropping them so that you can stay closer to the spawner.
https://minecraft.wiki/w/Monster_Spawner#Mechanicsyeah that ended up being the issue
Your player needs to be within the activation radius of the spawner. I don't know if the output is placed incorrectly.