I suspect you're going trying to partially update something that's hidden right? you can't do that as when it's hidden, it technically doesn't exist. So you need to partially update something that contains the hidden element but that always exists.
I did a show on this for NotesIn9... I forget which one. But when I typically do is if I have a panel that can be hidden I use an outer and inner panel. the outer panel is ALWAYS there, the inner has the formula. So I partial update the OUTER panel and everything works fine.
If you want to see this, it's probably in an early show of NotesIn9. Which are indexed at XPages.TV
Hope that helps!
Dave