If you, or some members, are not receiving the signup notices, check your Group Settings. In the left sidebar, under "Managers Only", open "Group" and click Group Settings. (For a community, open "Community" and click Community Settings.) Then go to the Notifications section.
Find the setting called Who should receive the signup notices?
The default setting is Only those who have not responded (less notices sent). Using this setting means if someone has already signed up, declined, or is on the waitlist or pending for that date before the notice is scheduled to be sent, then they will NOT receive that notice. This is the default setting because it results in fewer emails being sent to people.
The setting Everyone (if they already responded, they will get a confirmation) means everyone receives the notice, even if they have already responded. The subject line is adjusted to match what they have already done - for example it will say "Reminder" for someone who has signed up, "You can still sign up" for someone who declined, and "You're on the waitlist" for someone on the waitlist.