Orders not showing up

[theehman]

This has happened several times. I'll receive a Notification of Payment Received from Paypal but no actual order is generated by the site. There will be no order email telling me what was purchased nor will there be any record of the order in the Admin, although it will show that the customer did register an account with the site.
I had one customer that had this happen several times then suddenly his latest order completed as normal. When asked, he said he did nothing different from previous orders. I have had several orders from other customers not appear since then so it's not just him.
It gets very frustrating and looks unprofessional to email the customer and ask what they ordered. Has anyone else had this problem and possibly found a solution?