You might face a situation like, an alarm was violated but no alerts has been triggered. All alarms are working fine and sending mails even that particular alarm was working before couple of days..
The first and foremost things is you need to check, the number of notification sent for that particular alarm.
While creating an alarm you will configure the limit number of alerts for violation, based upon the configuration Down and Up alerts will be triggered.
When the violation occurs the down alert emails will be triggered based on the configuration. If it reached to the maximum limit it stops sending the alerts and it will not trigger mails for the new violations.
To resolve this you have couple of options:
1. Need to rectify the problem so that the problem will be resolved and the UP alert email will be sent also the Notification send will be reset back to 0.
2. Can Reset the alarm so that the Notification send will be reset back to 0.