HOW doesn't it work? How it fails to work can be a useful clue.
HOW is your report designed?
If you multiply two summary fields in a calculation field where calculation and summary fields are all defined in the same table and displayed on a layout based on the same table, you get a "grand total' type calculation where the values returned by the summary fields are based on the current found set.
Use Get Summary if you need to compute a sub total based on a group of records present as part of the current found set and then only if the "Break field" specified in the get summary calculation is part of the current sort order.
But there are many different ways to compute totals and subtotals besides the use of a summary field as described above.