ahhh yes it is a laying issue then I believe. and here is a big fat tutorial on the proper laying techniques and slicing in Fireworks. A guy that posts often in here and is quite the Fireworks Guru type, contributed to an article listed here...
Adobe Article: Taking a Fireworks comp to a CSS-based layout GordonMac
and yes.. if I were going to update an existing webpage design, I would take the entire layered and named png and update it as a whole so I could get my exact alignment on the slicing. I may only exprt a small portion of the sliced work though depending on what exactly it was I were changing.