What's causing this function to only run once?

I have a script for an npc AI to attack people, but it only functions once even though I have my key frames labeled in the animation. Does it have to do with the animation itself or is it the code? Any help fixing this would be appreciated as the game would be too easy without getting hurt by the enemies.

local animation_attack = Instance.new("Animation")
animation_attack.AnimationId = "http://www.roblox.com/asset/?id=736297595"
local running_animation_attack = nil

function attack (human)
	game:GetService("ContentProvider"):Preload(animation_attack.AnimationId)
	sound(181268614,linked_sword.Handle,math.random(90,110)/100,1,2)
	if(running_animation_attack == nil and human.Health ~= 0) then
		local event_keyframe_reached
		running_animation_attack = humanoid:LoadAnimation(animation_attack)
		running_animation_attack:Play()
		event_keyframe_reached = 	running_animation_attack.KeyframeReached:connect(function(keyframeName) -- Bind 	function to KeyframeReached event
			if (keyframeName == "Hit") then
				hurt(human)
			
			elseif(keyframeName == "End") then
				running_animation_attack:Stop()
				running_animation_attack = nil
				event_keyframe_reached:disconnect()
			end
		end)
	
	repeat wait(0.1) until running_animation_attack == nil
	end
end