Paypal order not received through Zen cart

Hello,
My client recently had an issue where a customer purchased a product through her Zen Cart store. My client received an email confirmation from Paypal only. There is no record of her purchase in the Zen Cart store. The only information that you can find about this customer is from when she registered. It seems that Zen Cart forgot about this transaction. Any suggestions? I am worried that this is happening without us even knowing. I have contacted Paypal but I haven't heard a response yet. Thanks to anyone who can help me figure this out. So strange.

For PayPal transactions using the "Website Payments Standard" (or "IPN") module, you must make sure a LOT of things are set up exactly perfectly, or else PayPal's remote notification to your store will fail, resulting in your store never seeing the transaction.
You can review setup and troubleshooting instructions here: http://www.zen-cart.com/wiki/index.php/PayPal

Or, you can use Express Checkout which is not hindered by the older technology that PayPal built their IPN/Standard architecture on.

She did get a confirmation from Paypal. The issue is that Zen Cart did not even see this transaction but Paypal did. It's like the order never existed in Zen Cart. Without the paypal receipt she wouldn't have known someone ordered a product. Any suggestions?

We plan on moving to Payments Pro.

PayPal will still send the email.
But if you're using the Standard/IPN module in your store, you MUST rely on PayPal's server doing a separate connection to your store in the background to tell your store about the order. I've already explained how to setup, troubleshoot, diagnose, and fix those problems.

I will go through the set up and troubleshooting. Thanks for your quick response. Is there a set up to follow for the Website Payments Pro? Thanks!

If you've subscribed to Website Payments Pro, you can enable that module after you've configured Express Checkout: http://www.zen-cart.com/getpaypal

After you've enabled Express Checkout, you'll want to click "Remove" on the Standard/IPN module.