Emails can sometimes be filtered into junk mail depending on your guests email settings. It is always worth asking them to double check.
Emails won't be sent to secondary email addresses in a guest group. While these email addresses can be used to login to the guest area from your website, all email correspondence will go to the email address of the lead guest. If you only have the email address of the secondary guest it is worth swapping them over so the email can be sent.
If you still experience problems please get in touch with our team who will be happy to look into this for you.