Hey there!
First of all, you have to absolutely ensure the emails were actually sent. Most probably they were (as customers are getting other types of email), but this is a basic step to identify the issues. To do that, please check the mail log on your server or request a mail log of the provider you're using (say, if you're sending your mail via Gmail).
Now, if the logs show an error, you have to fix it. If the logs say the email was sent successfully, there's another step.
Sometimes, emails don't even go to the spam folder and are rejected on the server level, and people never actually see them neither in their inboxes nor in the trash folders. This often happens for customers that have email accounts on Amazon, Yahoo, Microsoft etc.
So have a look at the emails of the people who never received the emails and check if they have email accounts from these companies. If this theory is valid your case, the best decision is to contact the support department of the company which provides email services and ask them to check if they blocked your emails, and if yes, then why.
Good luck with that!
