The autoresponse email is sent in the same way that the standard email is sent. If the standard email is working, the autoresponse should work too, but consider the points below.
Make sure you have enabled the autoresponse in the form configuration in the User Panel. The option to enable is Autoresponder
The autoresponder sends an email to the person filling out your form. It sends the email to the address entered in the email input on your form. For the autoresponder to work you should name your email input
<input type="text" name="email">
Make sure that the email address you are using for testing is good. Check that the email is not getting blocked by a junk filter. Remember this has nothing to do with the email address you configure for sending the standard email to.