I also have this problem (except I was in manual calculation). It has
happened intermittently in the past, but last week it became a real nuisance.

The cells contain genuine formulae, displayed as numbers and there are no
circular references. If I press Ctrl-Alt F9, they do recalculate (along with
every other formula), but this can take ages.

In one workbook, about 20 rows calculated in the middle of a sheet, but
those above and below (with identical formulae) did not. Pressing F9 or
changing calculation to Automatic had no effect. Somehow Excel seemed to have
lost the needing calculation flag for these cells.

I was able to save this book and someone else opened it, but there was no
warning that it had not calculated, nor did the cells calculate even after
they pressed F9. Cells would only calculate if you edited the cell and
pressed enter, or changed a precedent cell, or press Ctrl-Alt F9.

It is driving me mad!!!