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