It is probably due to the IPN payment script on your server. Check your server logs, and add additional logging to the script itself. Make sure the payment script is being executed, and that it's processing the data correctly.
Key Generation Fails Randomly
John MKeys are generated 100% reliably when the customers pay with credit card.About 60% of the orders paid by PayPal fail to generate a key even though the payment clears. This is not correlated with the country of origin US vs Non-US or Verification status of The Pay Pal account.Any suggestions? Thank you.