Plugin doesn't see the RSVP as paid

I have an event requiring payment. I paid to attend the event at PayPal and the event came back with this URL: http://www.deserthorticulturalsociety.org/events/2016/12/test-event-requiring-payment/?amt=10.00&cc=USD&charset=windows-1252&item_name=Test%20Event%20Requiring%20Payment&item_number=1231&st=Completed&tx=87X537206H8461441

But the plugin doesn't show the user is paid, and neither does the front-end code. I'm less concerned about the front-end code at the moment because of how much customizing I've done, but why didn't the plugin register the RSVP as paid?

Thank you