My spawn function have problems?

I'm trying to recreate flappy bird in roblox ,i made a function that will clone the pipes:

--here it will pick a random position with a random offset from -7 to 7 from the center of the screen
local function Center()
    local Num = math.random(6.5,20.5)

    return Num
end


-------------------------------------------------------------------
--here i made a function that will position the pipes as a pair and have a random offfset around the center Y coordinate
local function MakePillar()
    local newNum = Center()
    local points = {}
    local model = Instance.new("Model",game.Workspace.PillarsFolder)
    model.Name = "Pillars"

    local newPillar = Pillar:Clone()
    newPillar.Name = "newPillar"
    newPillar.Parent = model

    newPillar.Size = Vector3.new(3,newNum -Distance,3)
    newPillar.Position = Vector3.new(offset,newPillar.Size.Y/2 - Distance,0)
    points[1] = newPillar

    local newPillar2 = Pillar:Clone()
    newPillar2.Name = "newPillar2"
    newPillar2.Parent = model

    newPillar2.Size = Vector3.new(3,27 - newNum - Distance,3)
    newPillar2.Position = Vector3.new(offset,(27 - newPillar2.Size.Y/2)+Distance,0)
    points[2] = newPillar2

    return points
end

and here is the function that spawn the pipes and tween it:

local function tweenPart(part, sec, goal)
    local tweenInfo = TweenInfo.new(sec, Enum.EasingStyle.Linear, Enum.EasingDirection.Out)
    local tween =tweenService:Create(part, tweenInfo, goal)

    tween:Play()
end



--------------------------------------------------------------------
--Spawn Pillars
local function SpawnPillars()
    local Parts = MakePillar()
    for i=1, #Parts do
        tweenPart(Parts[i], 15, {Position = Parts[i].Position - Vector3.new(60,0,0)})
    end
end

until here everything work fine but then i put the spawn function in a loop and it will spawn pipes every 3.5 seconds, and it did work, but for some reason sometimes it spawns pipes in less than 3.5 sec, only sometimes, and not always, here is the script:

local function Pillars()
            while dead == false do
                SpawnPillars()
                wait(3.5)
            end 
        end

        spawn(Pillars)

Does anyone have any idea why this happen? Also here is the game link if u want to see the glitch, i believe u will understand what am saying after u saw it: https://web.roblox.com/games/5222604679/Flappy-Guy

Any help is appreciate, thankyou