Serial number not appearing automatically in the invoice text of the service line

In R11, whenever a new service line is created and the serial number of the product is entered in the sub-line, the invoice text (in the sub-line) shows the serial# with the proper start and end date.  But after we migrated to R12, serial# is not appearing by default, and we have to manually insert the same in the sub-line's invoice text.. Kindly let me know how we can solve this problem.