In Revit 2009:
If you are working on a big project, once in a while, the plan regions you had created before become toooooo tired and stop working. The irony is that they might have been working fine for the past 2 weeks, then suddenly they would behave as if they have the view range as that of the view itself.
Changing the view range in the plan region does not work. Neither changing the view range of the view itself. In the pic below, the window at 6' sill is not visible, even though the plan region is set to have the cut plane at 7'.
To correct this bug or memory issue, you have to select the plan region, and click edit in the option bar,
and then just move the plan region boundary by an inch (and you could move it back too!) and Finish sketch. Voila!
The window is back. However, if you have too many plan regions in the view, this process is painful. I hope someone comes up with a better solution.
(The other option is to send the file to Autodesk via FTP and wait for a couple of days....)