Why does my script not work at the elseif part?

game.Players.PlayerAdded:Connect(function(player)
    player.CharacterAdded:Connect(function(char)
    local leaderstats = player:WaitForChild("leaderstats")
    local CheckValue = leaderstats:WaitForChild(script:WaitForChild("leaderstats").Value)

if CheckValue.Value >= 0 and CheckValue.Value < 500 then

            if char:WaitForChild("Head"):FindFirstChild("Rank") then
                wait(3) 
                char:WaitForChild("Head"):WaitForChild("Rank"):Destroy()    
            end 

             if not char:WaitForChild("Head"):FindFirstChild("Rank") then
                local a = game.ServerStorage:WaitForChild("Ranks"):WaitForChild("NOOB"):Clone()
                a.Name = "Rank"
                a.Parent = char:WaitForChild("Head")
            end

    elseif CheckValue.Value >= 500 and CheckValue.Value < 1000 then

            if char:WaitForChild("Head"):FindFirstChild("Rank") then
                wait(3) 
                char:WaitForChild("Head"):WaitForChild("Rank"):Destroy()    
            end 

            if not char:WaitForChild("Head"):FindFirstChild("Rank") then                
                local b = game.ServerStorage:WaitForChild("Ranks"):WaitForChild("BETTER"):Clone()
                b.Name = "Rank"
                b.Parent = char:WaitForChild("Head")
            end

end

CheckValue.Changed:Connect(function()
if CheckValue.Value >= 0 and CheckValue.Value < 50 then

                        if char:WaitForChild("Head"):FindFirstChild("Rank") then
                            wait(3) 
                            char:WaitForChild("Head"):WaitForChild("Rank"):Destroy()    
                        end 

                        if not char:WaitForChild("Head"):FindFirstChild("Rank") then
                            local a = game.ServerStorage:WaitForChild("Ranks"):WaitForChild("NOOB"):Clone()
                            a.Name = "Rank"
                            a.Parent = char:WaitForChild("Head")
                        end

                    elseif CheckValue.Value >= 51 and CheckValue.Value < 150 then

                        if char:WaitForChild("Head"):FindFirstChild("Rank") then
                            wait(3) 
                            char:WaitForChild("Head"):WaitForChild("Rank"):Destroy()    
                        end 

                        if not char:WaitForChild("Head"):FindFirstChild("Rank") then                
                            local b = game.ServerStorage:WaitForChild("Ranks"):WaitForChild("BETTER"):Clone()
                            b.Name = "Rank"
                            b.Parent = char:WaitForChild("Head")
                        end

            end
        end)
    end)
end)

Why does this script only work for the top part, and not the "else if" part in line 48? So like, if you have more then the required level it still gives the lower rank...