Been having this problem for a while as well: https://forums.unrealengine.com/t/creature-spawners-bugs/2070227
It seems like with too many spawners they just start to break down, try removing some until it works how you want it to. A tip for lowering the number of spawners is to just have one spawner attached to a prop that teleports to where you want to spawn and spawns the creature/wildlife there.