Boms don't update

Hello;

       I would like to know, why when I make a change, to an assy., in that drawing for the assy., I always end up re-doing the Boms. Either the new part no., doesn't show up or there's some other little imperfection, that I don't know how to fix, other than deleting the bom, & putting in a new one. That takes care of it.  It just seems to me that this defeats the purpose of paremetrics in the software.

      Has anyone else noticed this type of behavior?

Thanks in advance, for any input;

Rob_D

SW 2013 Sp3

SolidworksDrawings And Detailing