If you are referring to the email notification for each new response submission, from my experience, most errors come from tenant settings such as firewall blocking of the sender from Microsoft, Exchange, or SharePoint service misconfigurations. To verify whether the issue comes from your tenant instead of Microsoft Forms, please check whether it's a user form or group form, and whether the accounts has been configured correctly to accept the sender outside of your tenant to send emails to those accounts. If you confirm there are no abnormalities in your tenant configuration, you can contact your IT department to raise a support ticket to the Product team, and they will work with you to find out more details.
