Vijay -- To do that sort of thing you usually need to have all the months in question in the underlying table for that sheet.
When you say "the last 12 months", is that based on "today"? Or is it based on the last date of data in your data source? If so, you can use LODs to grab sums of the last 12 months independent of any filters you have on the sheet.
Alternatively, you can use a parameter to let the user select a month, and then display only a portion of the underlying table based on the parameter.
Also, you can use a table calc as a filter to display the selected month. That will leave the underlying table intact, even when you only display one month. The other 11 months you need for the roll-up values would still be in the table to grab with another table calc.
At a high level, those are ways to do what you are describing. Without a specific workbook example, it's hard to give details on how to implement any of them.