if the formula manipulates the data in anyway, such as adding or subtracting it, then the aggregate functions won't work, since CR cannot summarize the data prior to display.
I know, sounds confusing. If you look at the data and you say if field1 = x then add field2 to a total, that can be summarized, but if you say if field1 = x then add field2 and if field1 = y then subtract field2, then CR isn't happy.
There is a way around this...use shared variables to keep track of your running total. Typically they come in 3's (reset, increment, display)
reset-group header
shared numbervar x:=0;
"" //hide the zero
increment detail section:
add to your existing formula
shared numbervar x;
existing formula
x:= x+commission;
commission;
display-group footer
shared numbervar x;
x
HTH