A question mark is FIleMaker's way of saying that it can't display a result in the field provided. It can be caused by a field that is too narrow to display the result (Try clicking in the field, if the value correctly displays when you click into it, this is probably due to the field being too small.)
Or it can be due to a math error such as dividing by zero or taking the square root of a negative number. Obviously, simple subtraction won't produce a question mark, but if either field is itself a calculation, that math error might be occurring in one of those fields and then you also get the ? here.
You can also get this ? if the calculation traps into an infinite recursive loop--that doesn't seem the case here, but again, that might be the case if Fee or account income Total have that issue.
And GetNthRecord calculations are a bit notorious for producing this result if you have a fairly large number of records in your found set with GetNthRecord referencing data from the preceding record in the found set.