After every successful payment, Paypal sends a notification for payment status. By default, you don't have to do any changes in Paypal. But due to certain reasons Paypal might not be sending notifications after payment. To enable it again, please follow the steps below:
1. Login to your Paypal.
2. Go to PROFILE-->Profile and Settings.
3. Click on MY SELLING TOOLS --> search for "Instant payment notifications"-->Click on "Update"
4. Click on "Choose IPN Settings" button.
5. In notification URL enter "http://www.appointy.com/PaymentProccessor/PayPalIPN.aspx" and select "Receive IPN Messages(Enabled) " and click on SAVE.
If everything is setup correctly, then please follow the steps outlined here to troubleshoot the IPN issue.