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