Some Alarms are not being sent as Emails, while others on the same site are being sent properly.
Some EventNotification objects were not configured properly to enable email on alarm.
Ensure that each EventNotification is enabled to allow alarms to be sent as Emails. There are two options that need to be enabled in each EventNotification.
1. Actions tab - Enable "Email on Alarm".
2. Delivery tab - In the Recipient, enable "Primary Email".