It's possible that the issue is that there is a dialogue open that isn't visible in Exposé (because they sometimes aren't), but is behind the front OmniFocus window. This happens far too frequently (in many, many different apps) when you use Spaces as well as Exposé in Leopard (especially if you haven't limited an app to a particular Space - the dialogue only ever appears in one, and invariably it isn't the one you are looking at, and when you do go to the one where it is located, it is also often now behind a window which you can't move or click into because there is a dialogue open... yes, this is very frustrating!).