If you, or some members, are not receiving the signup notices, check your Group Settings. You can find that under the Manage tab.
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 responds to an activity before the notice is scheduled to be sent, then they will NOT receive that notice since they have already responded for that date. This is the default setting because it results in less emails being sent to people.
The setting Everyone (if they already responded, they will get a confirmation) means that even if they have already responded, the member will receive a confirmation notice.