I'm not seeing any change in the GUI?

Basically, I created a "loading gui", which is a bar that slowly fills up, and then the GUI disappears after it's fully loaded.

There are no errors in the output, but the bar that's supposed to move, isn't moving.

Entire Script:

local gui = game.Lighting.IntroGui
local startvalue = 35

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game.Players.PlayerAdded:connect(function(player)
	wait()
	local guiclone = gui:clone()
	guiclone.Parent = player.PlayerGui
	local mf1 = guiclone.frame1.loadingframe.movingframe
	local text1 = guiclone.frame1.loadingframe.text
	wait(1)
	for i = 1,750 do
		local nextvalue = startvalue + 1
		wait()
		mf1.Size = UDim2.new(0, 0, 0, nextvalue)
	end
	text1.Text = "Finished Loading Assets!"
	wait(1)
	guiclone:Destroy()
end)

Section of the Script that isn't Working:

	for i = 1,750 do
		local nextvalue = startvalue + 1
		wait()
		mf1.Size = UDim2.new(0, 0, 0, nextvalue)
	end