If you enable Paypal, Simfatic Forms will process the form submission only after Paypal notifies the payment is successful. The Paypal notification is called IPN.
When the Paypal notification can't reach your website, the form processing can't complete. There can be two reasons why the IPN notifications are not working:
First, check your PayPal IPN history page:
Paypal IPN History Page
Check the error against each row. If the message says "disabled", the IPN messages are disabled in your preferences. You have to enable it back.
Go to: My Account -> Profile ->Selling preferences->Instant Payment Notification preferences In your Paypal IPN notification settings, leave the URL blank and enable "receive IPN messages(enabled)"
It is better to try with the paypal sandbox testing first.