PayPal "sandbox" mode works fine, switch to "live" produces error
I recently set up a Jobroller-themed website, with (3) job packs to start out. I set up a "PayPal Sandbox" to test order processing, and it works 100% smoothly. Orders are placed, paid for, email responses are delivered, and most importantly job orders are fulfilled and marked as "complete" and are listed accordingly in the Jobroller "Orders" tab. Even the multi-job pack properly gets assigned to the user with remaining "credits" for use. No problems occur in PayPal sandbox mode.
As soon as I switch to the "live" mode (by replacing the PayPal "sandbox" email address with my "real" PayPal Email address, and changing sandbox mode to "No") the order process fails. The job order is placed, and payment comes through PayPal, but I no longer receive the valid IPN message that I got in "sandbox" mode. Further, the job "Order" cannot be marked as paid, as this attempt returns an "Error: Invalid User Pack" response. This means that all of my job listings require manual "publishing" by an administrator, and the orders cannot be moved out of the "Pending" folder because they cannot be marked as paid. I'm unsure what to try to fix this, since everything works properly in sandbox mode.
Thanks for any assistance you can offer with this issue.