Hi Jen
The basic answer is that you can't do comparatives across short years.
As a workaround, I have added 24 columns to the far right of the statement that are set to "hide when print", spanning the prior 24 fiscal periods. Each column shows the net activity for a different fiscal period (using ColSpecs in the format of _[12]M and {1}_[12]M (check out the SuperSpec statement for how these work). Then in the Prior Year column I use a Sum formula to sum up the various periods I want to show in that column. It is not elegant and you have to change the range of columns in the Sum formula each month, but it is the best I've come up with.
Steve