Instead of making this an auto-enter calculation--which won't automatically update on existing records, make it a field of type calculation.
If you really want to stay with auto-enter, see this thread for a way to update your existing records: Updating values in auto-enter calc fields without using Replace Field Contents
Thanks, Phil. Changing the field to a field type of "calculation" made the difference. I don't quite understand why it allowed the result I want to display, but it does. And there is the added benefit of the existing records being automatically updated.
Many times, when we update an auto-enter calculation, we DON'T want the data in previous records to change. They may record a financial transaction and the change would change the total value of that transaction to a value that is not correct for past transactions.