It could be that you haven't activated IPN on the paypal side. Here's the steps you need to follow:

In order to use PayPal for automated subscriptions you must perform the following steps:

1. Log in to your Business or Premier account in Paypal.
2. Click the Profile subtab.
3. In the Selling Preferences column, click the Instant Payment Notification Preferences
link.
4. Click the Edit button.
5. Select the checkbox to activate IPN.
6. In the Notification URL text box, you can either leave this blank or enter the url to the paypal-ipn script, ie. https://www.ubbcentral.com/forums/ubbthreads.php?ubb=paypal-ipn
7. Click the Save button.