cannot say i do …..
i assume it worked before ?
nothing i have changed that would cause this to suddenly not work.
checked your/their junk folder / logs ?
does the order show up in the order history ?
might want to try to set it to sandbox / test, turn on logging (in the gateway settings), do a test order and see what happens and what the logs say (again in the wppizza-gateway-paypal/log/ directory)