This situation needs to be narrowed before a focus can be placed for troubleshooting and the best way to do this is to create a test reviewer with your email address. If you received all the notifications you intended to send, then the troubleshooting needs to be done on the reviewers side. Make certain that the notifications are not in the spam/junk email folder and also have them whitelist/add to safe senders list as this is the email address that the emails are being sent from.