prestashop order not showing in back end but customer paid

we have a prestashop site that occasionally is not showing order in the back office when a customer pays, what happens is the customer places the order in the cart then checks out and pays with paypal and we get the payment notification but there is no order in the back end? I need someone who is experienced in prestashop and with this particular issues as other have looked at it but they have not experenced the issue, Please do not bid if you do not know prestashop very very well i need someone with alot of experence with prestashop. thanks for your time