I have noticed that off late our Solarwinds Alert Engine Service not been sending alerts but the service status is Running.
when I restart the service, notifications are sent.
What could be that issue and how can I resolve this?
What notifications are you expecting? If the notifications were sent after a restart, that would be due to all the alerts being reset.
Do you alerts have the reset tab filled out? If not, then whenever an alert is made active (notification sent), it won't trigger again until it is reset.