Hi,
One more idea (guessing that you are working with web application) is that quite often developers make some web elements invisible by setting their size to zero or by moving them out of viewport.
Try to debug when such error occurs, get full name of the problematic object, find it in the Object Browser (manually navigating down the names hierarchy provided by the value of the FullName property) and check if this is really that object that you are expecting to work with. There are chances that somewhere around you may find a visible sibling. If this is a case, talk to your developers and ask them if they really 'hide' web elements instead of re-creating them. If they really follow this way and will refuse to change it (actually - this is a resource leak in the application), you'll have to search for required objects in your test code using more strict search conditions. (For example, search for the object that has width (or height) of positive value.)