Hello!

Problem may be in payment type. It can be PDT, IPN and direct. Looks like you have enabled IPN, in this case payment is performed but instead of post data with info to the site, payer gets email about successful payment only and this case there is not post data to the site, thus payment on the site can not be processed and register the product. 

We recommend to use PDT method, in this case payment will be process on the site also and don't need to back to the site with post data. But if you select PDT method in Paypal settings on the UNA site and don't enable it in paypal account, it will not work.

So, you need to check PayPal settings (Proccess Type) on the site and in your paypal account.