Random users not able to complete PayPal Advanced using iframe

I have a site that has been running for a while so it might be something PayPal has changed although I did update the site to MySQLi recently too.
It uses the PayPal Advanced checkout within an iframe.

They go to complete the checkout, enter their shipping info and click "Continue". Then the address form slides up, and the PayPal form is loaded where they can choose to either "Check Out with PayPal" or "Pay Now" using their credit card.

If they enter their credit card and click "Pay Now" it goes blank but never comes back with a message. Not the whole page, just the area where the iframe should appear, goes blank. I realize that at that point they should see either the success page or the failure page. Their order never goes through when the column goes blank.

I can't seem to reproduce the issue myself. It always works for me when I checkout and others checkout just fine. But there has been enough people that have experienced this, that I know it's not just one instance of user error.

Also, I have confirmed with at least one user that they are doing this on a desktop PC and have tried Internet Explorer and Chrome with the same results.

Any idea where I should begin? Could this be a javascript issue or something else?

Thanks for any ideas you might have.
TroyD