We can see event count got gradually decreased for last two months so we wanted to check the root cause for this.
Unknown. But likely culprits are in teh resolution.
All APM Releases.
1) Check overall historical metric graph performance. See what dates/times would have generated alerts.
2) Test sending multiple Alerts and see if sent in EM log. If so, then likely not an APM issue.
3) Check mail relays, SNMP once leaving EM.
4) See if the Agent is down during that time.