ZIndex changing value in script doesn't work?

I have a script that uses tweening to make the stats of a gun visible while the gun hovers over it, and the script works, but by using multiple UIs it's shown as behind of the UI that's next of the mouse hovering UI.

I've decided to use ZIndex to change the values of the UIs while the cursor hovers, but the ZIndex keeps the same value and it doesn't change. I've wrote the ZIndex changing values like the example shown below.

local sliding = script.Parent.Stats
--local symbol = script.Parent.HideBar.Symbol	

b=true

function OnMouseHover()
b=false
	sliding:TweenPosition(UDim2.new(0,140,0,0),"Out","Linear",0.25,true)
	--symbol:TweenPosition(UDim2.new(0,0,-0.5,0),"Out","Linear",0.5,true)
	
		--ZIndex Changing

		sliding.Damage.ZIndex = 2
			sliding.Damage.Symbol.ZIndex = 2
			sliding.Damage.Fill.ZIndex = 2
		sliding.Firerate.ZIndex = 2
			sliding.Firerate.Symbol.ZIndex = 2
			sliding.Firerate.Fill.ZIndex = 2
		sliding.Movement.ZIndex = 2
			sliding.Movement.Symbol.ZIndex = 2
			sliding.Movement.Fill.ZIndex = 2
		sliding.Zoom.ZIndex = 2
			sliding.Zoom.Symbol.ZIndex = 2
			sliding.Zoom.Fill.ZIndex = 2
		sliding.GunType.ZIndex = 2
		
		script.Parent.Background.ZIndex = 3
		script.Parent.GunImage.ZIndex = 5
		script.Parent.RarityBar.ZIndex = 4
		script.Parent.GunTitle.ZIndex = 3
		script.Parent.SkinTitle.ZIndex = 3
		
	if b == true then
		sliding:TweenPosition(UDim2.new(0,0,0,0),"Out","Linear",0.25,true)
		
		sliding.Damage.ZIndex = 1
			sliding.Damage.Symbol.ZIndex = 1
			sliding.Damage.Fill.ZIndex = 1
		sliding.Firerate.ZIndex = 1
			sliding.Firerate.Symbol.ZIndex = 1
			sliding.Firerate.Fill.ZIndex = 1
		sliding.Movement.ZIndex = 1
			sliding.Movement.Symbol.ZIndex = 1
			sliding.Movement.Fill.ZIndex = 1
		sliding.Zoom.ZIndex = 1
			sliding.Zoom.Symbol.ZIndex = 1
			sliding.Zoom.Fill.ZIndex = 1
		sliding.GunType.ZIndex = 1
		
		script.Parent.Background.ZIndex = 2
		script.Parent.GunImage.ZIndex = 4
		script.Parent.RarityBar.ZIndex = 3
		script.Parent.GunTitle.ZIndex = 2
		script.Parent.SkinTitle.ZIndex = 2
		
		--symbol:TweenPosition(UDim2.new(0,0,0,0),"Out","Linear",0.5,true)
	end
end

script.Parent.MouseEnter:connect(OnMouseHover)

function OnMouseNotHover()
b=true
	sliding:TweenPosition(UDim2.new(0,0,0,0),"Out","Linear",0.25,true)
	
		sliding.Damage.ZIndex = 1
			sliding.Damage.Symbol.ZIndex = 1
			sliding.Damage.Fill.ZIndex = 1
		sliding.Firerate.ZIndex = 1
			sliding.Firerate.Symbol.ZIndex = 1
			sliding.Firerate.Fill.ZIndex = 1
		sliding.Movement.ZIndex = 1
			sliding.Movement.Symbol.ZIndex = 1
			sliding.Movement.Fill.ZIndex = 1
		sliding.Zoom.ZIndex = 1
			sliding.Zoom.Symbol.ZIndex = 1
			sliding.Zoom.Fill.ZIndex = 1
		sliding.GunType.ZIndex = 1
		
		script.Parent.Background.ZIndex = 2
		script.Parent.GunImage.ZIndex = 4
		script.Parent.RarityBar.ZIndex = 3
		script.Parent.GunTitle.ZIndex = 2
		script.Parent.SkinTitle.ZIndex = 2
	
	--symbol:TweenPosition(UDim2.new(0,0,0,0),"Out","Linear",0.5,true)
	if b == false then
		sliding:TweenPosition(UDim2.new(0,140,0,0),"Out","Linear",0.25,true)
		
		sliding.Damage.ZIndex = 2
			sliding.Damage.Symbol.ZIndex = 2
			sliding.Damage.Fill.ZIndex = 2
		sliding.Firerate.ZIndex = 2
			sliding.Firerate.Symbol.ZIndex = 2
			sliding.Firerate.Fill.ZIndex = 2
		sliding.Movement.ZIndex = 2
			sliding.Movement.Symbol.ZIndex = 2
			sliding.Movement.Fill.ZIndex = 2
		sliding.Zoom.ZIndex = 2
			sliding.Zoom.Symbol.ZIndex = 2
			sliding.Zoom.Fill.ZIndex = 2
		sliding.GunType.ZIndex = 2
		
		script.Parent.Background.ZIndex = 3
		script.Parent.GunImage.ZIndex = 5
		script.Parent.RarityBar.ZIndex = 4
		script.Parent.GunTitle.ZIndex = 3
		script.Parent.SkinTitle.ZIndex = 3
		
		--symbol:TweenPosition(UDim2.new(0,0,-0.5,0),"Out","Linear",0.5,true)
	end
end

script.Parent.MouseLeave:connect(OnMouseNotHover)

Why doesn't the ZIndex change?

If this is a serverscript, then you cannot access the playergui or any clientsided-related stuff. You need to use a localscript in order to do so.

Unless you're using a localscript already: Changing the ZIndex shouldn't be an issue.