Potential Set Analysis Issue

[#1]

Hi

This might not be a set analysis question.... It probably my naivety!

I have some data as below:

And I'm trying to Report Current Month chosen, prior month and prior year, as below!

I have created some variable to help me get to this point:

But i'm getting some issues - If i leave my choices empty it's all good (it defaults to the max, asin the screen shot below) - the minute I choose a year and a month from the selection, the Prior Month figure goes to zero... I suspect its as there is no longer data for months other than the ones I've chosen in the data set - there obviously is in the source data....  If this is the case how can I solve the issue?

The expressions is below:

Prior Month: =Num(Sum({<  DataYear = {$(=vPriorMonth_Year)} , DataMonth = {$(=vPriorMonth_Month)}     >} NewBusinessInMonth),'#,##0')

The data is month by month - ie a Monthly position has been derived in the back end database, so I am comparing month on month!

Cheers