Why does this Double Jump fail sometimes?

I'm not entirely sure what is wrong with your code, but here is basically the only improvement that could fix this problem;

UserInputService > KeyDown

KeyDown is deprecated, so it can be removed at any time. It may also not work properly sometimes. Instead, you should use UserInputService.

game:GetService("UserInputService").InputBegan:connect(function(key,gameProcessedEvent)
	if key.KeyCode == Enum.KeyCode.Space then -- it might be SpaceBar or something, I don't have access to KeyCodes at the moment
		-- code
	end
end)

It is very similar to KeyDown, just that the returned key is not a string, but an Object with a property of 'KeyCode', and a second parameter - whether the game processed an event (like when a player is typing, so they don't jump when they press space)

If you're still confused, let me compile the code into your new script:

local Player = game.Players.LocalPlayer

while not Player.Character do
    wait()
end

local character = Player.Character
local mouse = Player:GetMouse()
local humanoid = character:WaitForChild("Humanoid")
local torso = character:WaitForChild("Torso")
local head = character:WaitForChild("Head")
local debounce = true

game:GetService("UserInputService").InputBegan:connect(function(key)
    if key.KeyCode == Enum.KeyCode.Space then                                                            
        if humanoid.Jump == true then                                               
            if debounce == false then 
                return 
            end
            debounce = false
            local torso = character:FindFirstChild("Torso")
            torso.Velocity = Vector3.new(torso.Velocity.X, humanoid.JumpPower * 1.7, torso.Velocity.Z)
        end                                     
        repeat wait() until humanoid.Jump == false and humanoid:GetState() == Enum.HumanoidStateType.Landed
        debounce = true
	end
end)

Also use 'tab' to format your code, not space.

Hope I helped :)

~TDP