I added a Response page and the students say they've replied, but I've received no responses.

First, be sure that the registered email address in your web is valid.  Try sending yourself an email from your web.  If you don't get your test email, then you may need to update the email address in your web (instructions here).

Alternatively, since the Response is sent as an Attachment, be sure that if you apply Spam filters to your email system that that does not cause it to reject emails with Attachments. You can test this by temporarily removing the Spam filter and sending a response. 

If you are using a school email address, you may also need to check that your school or district email system does not have a spam filter in place that is rejecting messages.

You should also check your Response page's Update page, and look at the Options section at the bottom.  If you have selected the check box for accumulating responses to be emailed later, this means that all your responses are collecting in a file online, and are not being sent individually.  In this case, you should click the Retrieve All Responses button.  When you do this, you will be sent a single email containing all responses that have been submitted since you activated the option (or since you last clicked the Retrieve All Responses button).