Are the zombies getting stuck somewhere on the way to you? I had a 20 block deep pit with spikes surrounded by concrete pillars and was only getting maybe 10 zombies falling down it all horde night. Turns out they were getting stuck behind a big rock on the way to the pit entrance and just sat there all night beating on it. Went on for like 5 horde nights in a row. Once I cleared a path to the pit it started working. I think once a certain number have spawned on you it won't spawn more until those are dead.
