Form uses the standard wp_mail() function to send email…
So, it is likely the email address that the notification email is being sent to is blocking it or the email client is putting it in the spam folder.
I suggest de-activating the SMTP plugin, that can complicate setup… and then try changing the email addresses that the notifications to be sent to, to Gmail or Hotmail accounts. I suggest them because I know thru testing that they do not block the notification emails.
Please let me know how you make out.