Occasionally I run into this problem:  when working with an involved
spreadsheet with numerous defined names and ranges, Excel fails to
update.  This often happens when I try to change one of the
definitions:  let's say, for example, the discount rate, on which a
whole host of cells depend, is increased from 10% to 12%.  This will
go through several rounds of iterations to recalculate

At this point, however, Excel reverts to the calculation mode and I'm
forced to press F9.  But it still fails to update the variable in
different worksheets.  I usually exit out of Excel or reboot
altogether and the problem has always goes away, especially with the
rebooting.

But it's is happening too often.  Does Excel automatically revert to
Manual Calculation if it's overwhelmed?  I never put my spreadsheet in
that mode.  How can I keep this from happening when using complex
financial models?  Thanks.