Something is preventing your store from receiving PayPal's order notifications, or from completing the save of those details when received, thus they're not showing in your store.
It could be a result of a bad PayPal configuration, a bad firewall configuration at your hosting company, a bad server configuration for your site, a bad upgrade of files on your part, etc. The issue is discussed in multiple places around the forum.
You'll have MUCH LESS trouble with PayPal transactions if you TURN OFF the IPN/Standard module and turn ON the Express Checkout module instead. It's easy to configure and far more reliable: https://www.zen-cart.com/getpaypal
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