Ratheesh,
Please make sure that you are using the actual field names and not the display names in your code (you can find these in the field properties in the Administration Client.)
Also, ensure that the fields you are working with are in fact attached to the type for the workitem and are set as a visible field.
When debugging, did you check that you were getting the workitem ID correctly? If this was coming back with bad values, it could also cause the fields to not be found.