I have a spreadsheet with a single forumla that is not updating automatically. All other formulea on the sheet update automatically. Anyone seen anything like this?

The specifics: Time sheet spreadsheet, bosses want to track billable vs. unbillable hours automatically. I set up a dummy page that captures (via IF statements) the unbillable hours based on the Department number (7 nos. for billable, 1 for unbillable). I then sum those unbillable hours and refer to that summation from the main page. All that works fine. I then have a cell with formula subtracting the Non-billable hours from the total hours (thus, billable hours). THAT formula will not calculate without a little prompting.

Any ideas?