My next page gui won't work?

Explanation

My gui is made to switch between 2 pages when someone clicks next/previous. This is just added on to my shop gui, which isn't relevant.

Problem

This works, but for some reason I need to click twice when switching between pages. This can cause confusion, and people may think it's broken.

Code [First is for the next button, and the second is for the previous button.]

npopen = false

script.Parent.MouseButton1Down:connect(function()
    if npopen == false then
        npopen = true
        script.Parent.BackgroundColor3 = Color3.new(100/255, 100/255, 100/255)
        script.Parent.Parent.Previous.BackgroundColor3 = Color3.new(0, 0, 0)
        script.Parent.Parent.NextPage.Visible = true
    elseif npopen == true then
		npopen = false
        print('already open')
    end
end)

local popen = false

script.Parent.MouseButton1Down:connect(function()
    if popen == false then
        popen = true
        script.Parent.BackgroundColor3 = Color3.new(100/255, 100/255, 100/255)
        script.Parent.Parent.Next.BackgroundColor3 = Color3.new(0, 0, 0)
        script.Parent.Parent.NextPage.Visible = false
    elseif popen == true then
		popen = false
        print('already closed')
    end
end)

Links

http://prntscr.com/7mcvak

http://www.roblox.com/shopthing-item?id=263133147

Your problem is very simple. When you set your isNext variable, you already put .Value. Therefore, it's going to save the value what it currently is. In other words, it won't keep updating the variable, whenever the BoolValue changes. The way to fix this is by removing .Value from where you declare the isNext variable, and whenever you're comparing it or setting it as a new value, add .Value. Here are your scripts:

wait(2)
local isNext = script.Parent.IsNext

script.Parent.InputBegan:connect(function(input)
    if input.UserInputType == Enum.UserInputType.MouseButton1 then
        if isNext.Value == true then
            script.Parent.BackgroundColor3 = Color3.new(100/255, 100/255, 100/255)
            script.Parent.Parent.Previous.BackgroundColor3 = Color3.new(0, 0, 0)
            script.Parent.Parent.NextPage.Visible = true
            isNext.Value = false
        elseif isNext.Value == false then
            print('Already open')
        end
    end
end)

wait(2)
local isNext = script.Parent.IsNext

script.Parent.InputBegan:connect(function(input)
    if input.UserInputType == Enum.UserInputType.MouseButton1 then
      if isNext.Value == false then
            script.Parent.BackgroundColor3 = Color3.new(100/255, 100/255, 100/255)
            script.Parent.Parent.Next.BackgroundColor3 = Color3.new(0, 0, 0)
            script.Parent.Parent.NextPage.Visible = false
            isNext.Value = true
        elseif isNext.Value == true then
            print('Already closed')
        end
    end
end)