don't give up Becki, it's probably something simple 
as a first try, go to the File menu, then Report Options then ensure that both of the Convert...Null options are checked. if there's a Null value coming from the database then that will be the culprit causing your formula to fail.
or...if you don't want to convert nulls ('' for null strings, 0 for null numbers) then you have to use an 'if isnull()' check on your fields in the formula. the first method is easier though. if you do want this option, please send us the code in your formula.
cheers,
jamie