DrByte:
What happens if you run the ipncheck tool? http://www.zen-cart.com/forum/showthread.php?t=65680
I had a same problem.
I lost two orders in 2 last weeks, before was working fine(no order lost).
recently, 10 paypal orders, lost 1.
I received the paypal payment, but no order in the zen-cart.
Customer check out and pay via paypal, but the items still in their shopping cart, the system didn't create the order actually.
I check the ipncheck.php , the result is:
IPNCHECK.PHP - Version 1.0
IPN TESTING - POSTING to PayPal via: ssl://www.paypal.com:443
IPN TESTING - Confirmation/Validation response: RESPONSE RECEIVED - Communications OKAY
Script finished.
it happened randomly, it's hard to test it. :(