Why? because the PayPal IPN/Standard relies on waiting for PayPal's server to find a way to communicate to *your* server to save the completed order.
If *anything* goes wrong in that process, the order will never show in your store.
Options:
a) Change to using Express Checkout instead, since it doesn't have all the technical dependencies that IPN does. *This is the RECOMMENDED approach*.
Express Checkout is FAR more reliable.
b) and/or fix all the possible communication problems between PayPal's server and yours. Some tips can be found here: http://www.zen-cart.com/wiki/index.php/PayPal




