Hello I am very new to this craft and I've always wanted to make a zombies map. I'm sorry to be a bother and this has probably been asked 1 million times, right now I've made a little test map and I'm having promblems with hellhounds and they won't spwan in I hear the sounds but they won't spwan in I looked at this tutorial (https://youtu.be/poJyB2BKji8 tutorial here) and I did everything he said but nothing happens if anyone can help me that would be awesome

Have you added the "info_volume" with the followings kvps:
targetname = start_zone
target = start_zone_spawners
classname = info_volume (This one is automatically created)