If your client is notifying a group, and a couple of members to not receive that notification, you will want to check out those particular members group settings. To do so, you or an organization Admin need to go to Admin > Users & Roles > Users >
[select the user]. Go to their Groups section and make sure the check box for "Included in notifications" is checked.
Please try this for your users who are not currently receiving the group notifications and see if it fixes the problem for them.