PayPal Single Payments - can't get subscription to activate

I cannot seem to get a subscription to activate when using PayPal single payments. The payment goes through and the user is sent back to the site, but the purchased subscription is not activated. The user's account is created, but with no subscription.

I have found conflicting information in the forums about the PayPal IPN and have tried having it both off and on, but no luck. I have temporarily disabled the plugin until I can get this part working.

Am I missing a setting, perhaps? Wouldn't a subscription be added to a user account once the payment has been processed successfully? Any help you can provide is greatly appreciated. Thanks!