What to do when a formula is entered correctly (and works in some cells),
however, it does not work in others.

Example:
5 columns; Year 1 Budget (a), Year 1 Expenditures (b), Year 2 Budget (c),
Year 2 Expenditures (d), Total (e) (assuming this is line 1)

the equation in column e should be:

=sum(a1-b1+c1-d1)
This works in most of the cells - others will not "take" the equation and
revert to "0"