Why doesn't ProcessReceipt Fire?

I've tried everything I could think of, but the function won't fire when I buy the dev product. sometimes after I rejoin the place, it recognises that I bought the item, but I'd like it to recognise instantly.

the place with the script in question is here

and here is the script in question, all variable references are to existing items

if game.ServerScriptService:FindFirstChild("VaporPurchaseHandler") ~= nil then
	script.Parent = game.ServerScriptService
else
	script:Destroy()
end

-- debugging code added by ihaveamac
enable_debug_print = true
function print_d(m)
	if enable_debug_print then
		print("<debug> "..m)
	end
end

print_d("DEBUG MODE ACTIVE")

local MarketplaceService = game:GetService("MarketplaceService")
local PurchaseHistory = game:GetService("DataStoreService"):GetDataStore("PurchaseHistory")
local data = game:GetService("DataStoreService"):GetDataStore("VaporData")

print_d("psready")
 
MarketplaceService.ProcessReceipt = function(receiptInfo) 
	print_d("purchased")
    local playerProductKey = receiptInfo.PlayerId .. ":" .. receiptInfo.PurchaseId
    if PurchaseHistory:GetAsync(playerProductKey) then
        return Enum.ProductPurchaseDecision.PurchaseGranted
    end
	print_d("RUNNING: for i, player in ipairs(game.Players:GetPlayers()) do")
    for i, player in ipairs(game.Players:GetPlayers()) do
		print_d("RUNNING: if player.userId == receiptInfo.PlayerId then")
        if player.userId == receiptInfo.PlayerId then
				print_d("condition successful")
	
				local vapor = player.PlayerGui:FindFirstChild("Vapor")
				local props = vapor.Properties
				local GameId = props.ProductId.Value
				local suffix = props.DataSuffix.Value
				local main = vapor.Window
			
            if receiptInfo.ProductId == GameId then
				print(player.Name)
				print(main.Name)
				main.Buy.Visible = false
				main.Load.Visible = true
				print_d("gameIdAffirmative")
				if main.Buy:FindFirstChild("Gift") ~= nil and main.Buy:FindFirstChild("Gift").isGift.Value == true then
					print_d("isGift")
					local text = string.lower(main.Buy:FindFirstChild("Gift").GiftBox.Text)
					local giftlist = data:GetAsync("giftlist"..suffix)
					if giftlist ~= nil then
						local gifted = false
						for i = 1,#giftlist do
							if giftlist[i] == text then
								print_d("already gifted to "..text)
								gifted = true
							end
						end
						
						if gifted == false then
							print_d("gifted")
							table.insert(giftlist,text)
							data:SetAsync_d("giftlist"..suffix,giftlist)
							main.Load.Text.Text = "Gift Sent.."
						end						
						
					end
					
					wait(1)
					main.Load.Text.Text = "Processing.."
					main.Load.Visible = false
					main:FindFirstChild("Buy").Visible = true
				
				elseif main.Buy:FindFirstChild("Gift").isGift.Value == false then
					
				print_d("notgift")
                data:SetAsync(player.userId..suffix,true)
				main.Load.Text.Text = "Purchase Complete.."
				wait(1)
				main.Load.Text.Text = "Processing.."
				wait(1)
				main.Load.Visible = false
				main.Buy.Visible = true
				main.Visible = false
				vapor.Back.Visible = false
				print_d("buydone")
				
				else print_d("notgift, and not notgift")

				end
            end
		else
			print_d("condition failed")
        end
		print_d("if statement completed")
    end
    PurchaseHistory:SetAsync(playerProductKey, true)	
	print_d("purchasefinish")
    return Enum.ProductPurchaseDecision.PurchaseGranted	
end

print_d("purchase handler is ready")