I go this to work 
I added the call to check_changed_data inside the program instead of calling the same within the class. This way I check if the data is changed if yes, overwrite the public attribute of the class and then call the class to perform the required changes based on the changes made at the ALV level.
Although this method works, can anyone share any insights on why the changed data method does not show the changes when called from the class instead of the program?
Thanks,
Vikram.M