If you look in your Mals account under List Orders and click to see details of an order and see "No confirmation of payment has been received from PayPal for this transaction.", this means one of three things could be the cause. Either no payment was actually made through Paypal for this order, a payment was made by echeck through Paypal which hasn't cleared yet or Paypal's relay simply didn't make it back to the cart.
Double check your Paypal account to make sure you actually received payment. Make sure you set up auto return in your paypal account
- Access your PayPal account and select the My Account tab.
- Click the Profile subtab.
- Click the Website Payment Preferences link in the Selling Preferences column on the right.
- Click the Auto Return On radio button to turn on Auto Return
- Return URL is your website URL
- Turn off all other options