BuddyPress.org

Are you sure it hasn’t been hijacked by another theme? For instance, let’s say I put a widget in a 3rd column. Then, I switch to a 2 column theme. Everything that was in that 3rd column seems deactivated but really it’s just hiding because it thinks it’s already in use somewhere. But since my 2 col theme doesn’t have that 3rd one, I never see it.

Even if both of the themes are 3 column, if the sidebars are named differently, the widgets get hijacked. If this is the case, you need to go back to the old theme, get the widget out of the sidebar and THEN change to the new theme.