Hi there,
I think this is a problem relating to the coupon module, but it's possible the PayPal code could also be causing the problem.
We have a store with loads of orders that pay via Paypal. All of these complete OK.
However, with a coupon discount applied at the checkout, they pay with PayPal and the orders sticks on Payment Recieved - it does not update to complete.
I've also posted on the coupon issue queue on Drupal.
Thanks,
David