I got the solution to some extent.
Approving Manager (1-up manager) was not receiving notifications
because there was stack of entried in tables T71ADM_EE_HIST and
T71ADM_PROCESS. After deleting records from this tables, once manager
submits all his direct report's planning a notification is sent to
approving manager.
Now, once approving manager approves or rejects all the employee
plannings under one single manager then manager is not getting any
notifications.
i.e. workflows WS04000023 and/or WS04000024 are not triggering once
approving manager approves all employees planning under one single
manager.
Event linkage, making task as general task etc all basic configurations
are in place.
-Pradeep