As suggested we have restarted ERA Agent by logging individually in different hosts, this worked well in case of Virus signature database update as the Product Activation client task has been executed well and the Issue got resolved.
But this not the same case when we have initiated Product activation client task for the hosts which had last connected more than one month time span were gathered and have individually restarted ERA Agent services by logging into different hosts. After initiating wake up call, Product activation client task has been pushed. I See there is no change in the last connection time for these hosts even after agent services are restarted. And in the client Task i see these hosts under planned with no status(started/Successful/Failed) since last seven days.
I could not able to find what is the root cause of these hosts why they are not getting connected since long time, I have initiated Product activation client task in particular as I have seen product activation was failed in the task history of hosts triggered in common.