Can you check log in admin ? if you found something like
The remote server returned an error: (403) ForbiddenI would refer you
hereIf you do not see any errors make sure you configure settings correctly
You have to Enable auto return and set PDT url in paypal
1. Log in to your PayPal account.
2. Click the Profile subtab.
3. Click Website Payment Preferences in the Seller Preferences column.
4. Under Auto Return for Website Payments, click the On radio button.
5. For the Return URL, enter the URL on your site that will receive the transaction ID posted by PayPal after a customer payment (http://www.yourStore.com/Plugins/PaymentPayPalStandard/PDTHandler).
6. Under Payment Data Transfer, click the On radio button.
7. Click Save.
8. Click Website Payment Preferences in the Seller Preferences column.
9. Scroll down to the Payment Data Transfer section of the page to view your PDT identity token.