Running Total

<p>I am developing a report where I use formulas to calculated database data and want to create a manual running total on that but for some reason it will not let me select or even show the formulas in the picklist to use for running totals. Even selecting the formulas and using the popup to select insert summary does not give me the option to use these fields to create a summary on.</p><p>Is there a catch here?</p>