Greetings Ganesh,
The following prerequisites must be met:
- the maintenance plans are scheduled.
- either the maintenance order or service order is set as the maintenance call object for the maintenance plan category.
- the maintenance plan has neither the status inactive nor has a deletion flag been set.
- the times for the operations are maintained and/or materials with prices are assigned to the operations in the maintenance task list.
See this thread http://scn.sap.com/thread/3717711