Udim2 value resets, but doesn't decrease anymore, can anyone help?

So, I did watch a tutorial on how to use udim2 and I messed around and I watched videos. But I've been tweaking with it by myself now. I've noticed when I kill this "boss" the health bar disapears. When it reappears, the udim2 doesn't work anymore. The health bar stays stuck. Please help me

wait (.1)
while true do
	script.Parent.Parent.Parent.Visible = true
	local hp = game.Workspace.Boss.Humanoid.Health/300
	script.Parent:TweenSize(UDim2.new(hp,0,1,0),Enum.EasingDirection.Out,Enum.EasingStyle.Quad,0.15)
	wait (0.1)
	if hp < 0.01 then
		script.Parent.Parent.Parent.Visible = false
		wait (10)
		script.Parent:TweenSize(UDim2.new(0,647,0,70))
	end
	wait (.2)
end