Why doesn't my value increase when someone purchases?

Hello!

I am trying to get my leaderstat "Purchases" to increase when a player buys something in my store:

local mpService = game.getService("MarketplaceService")

mpService.ProcessReceipt = function(purchaseInfo)
    local plr = game.getService("Players"):GetPlayerByUserId(purchaseInfo.PlayerId)
    if purchaseInfo.ITEM_ID == 123456 then
        --The value that will be incremented
        local Purchases = plr.leaderstats.Purchases;
        Purchases.Value = Purchases.Value +1
    end
    return Enum.ProductPurchaseDecision.PurchaseGranted
end

I put 123456 as a placeholder, but it would be the actual ID of the clothing item being purchased.

Any help would be appreciated, TY