The problem is that running totals don't always work in headers. You don't have the count until the group is completed, so it has to go in the footer.
However, there is a way around this using two formulas instead of a "running" total.
In the first formula, which I'll call "CompleteEarly", but the formula you used under "Evaluate":
if {database field for date completed} <= {database field for date due} then
1
else
0
In the second formula, put the following:
Put this formula in your group header to get the info you're looking for.
-Dell