Paypal Express Checkout problem

I have a problem that only occurs when the buyer checks out through the Paypal Express Checkout but pays directly through their own Paypal account. The problem is that buyer gets the Transaction Failed page and, of course, the receipt is not generated. However, the transactions are going through successfully. I can see the transaction went through successfully on the Paypal account and I can see the summary and details on the database.

Paypal Express Checkout works fine though when the buyer fills the Paypal form. The problem may have a simple fix I need some direction. What could be causing it? I will attach the file on the next post if you need to see it. I thought maybe with some simple suggestions I can figure it out.

Thank you.