To verify this happen due to reaching the limit in notification policy, you need to create an alert history report as follows:
1- Select Auditing & Alerting | Alerts.
2- Select Alert History | Report.
3- By default, all dates are included in the report.
• To specify a specific day for the report, select Date, and type a date or select from the calendar.
• To specify a range of dates, select Date Range, and type the dates or select from the calendar.
4- By default, all alerts are included in the report. To filter the report, select Filter by Alerts, and select specific alerts to include in the report.
5- Type a name for the report.
6- Choose a format for the report. By default, a PDF file is created.
7- By default the report is generated and sent by email to the listed recipients. By default, the logged in account displays in the Email Addresses list. You can add more addresses to receive the report by email. A default subject line is included. Set the priority of the email.
8- You also can save the report to a specified location on the Save to Folder tab. Add a path to the location where you want to store the report file.
Review the report and check if you can find the missing alert in the report. Once you find the alert you will see the "Failed (Limit Reached)" comment in front of the alert.
Check notification policy settings and change/disable it as required.
To change the notification policy, follow the steps below:
1- Select Auditing & Alerting | Alerts.
2- Click Notification Policy.
3- Click Settings.
4- By default, the alert notification policy is enabled. To disable the policy, clear the check box.
5- Set the maximum number of alerts to send and the period of time to include. By default, a maximum of 100 alerts are sent in a 20 minute period and the counter resets after 10 minutes.
6- By default, a notification is sent to the administrator when alerts are suspended. To not send notifications, clear the check box.
7- Click OK.