Filter Actions only 1/2 way work from Grand Totals and Subtotals. If the Filter Action is changing focus (like to another dashboard) then that action is taken, however the Filter Action doesn't actually filter anything. This is unfortunately a "feature" of Grand Totals and Subtotals.
There are at least three workarounds to this:
1) Teach your users to click on the headings instead of the Grand Totals. For example if I click on Bookcases instead of the #45 then I see all 45 rows for bookcases in Dashboard 2.
2) Do a self-union of the data source and build your own custom grand totals as described in http://drawingwithnumbers.artisart.org/customizing-grand-totals-part-3/, then with the BYO grand totals you'd be able to use a Filter Action since they would be regular marks.
3) If the number of values of the dimensions on X and Y (months and product sub-categories in this case) isn't so large that the view would need scroll bars then you could make the grand totals as separate worksheets on a dashboard and then you'd be able to use Filter Actions.