Why icmpping does not trigger when server is power off ?

[bab]

I have created ping item on my host with follow key :
icmpping[172.20.20.20,4]
and have created a trigger such as follow :

last(/HOSTNAME/icmpping[172.20.20.20,4])=0
or
nodata(/HOSTNAME/icmpping[172.20.20.20,4],2m)=1

Now my host is power-off but trigger does not fire and does not show any alert for that and just is showing Zabbix agent is not available (for 3m)
What is the issue ?