Pivot table grand total not working - help!

Hello,

I have an ordinary pivot table based on data which includes the following information:

Opportunity name, Margin, Probability(e.g. 40%, rather than 0.4), Invoice month

And in my pivot table I am summarising these per month, with the value fields as a calculated field which is (Probability/100)*Margin.

The table entries are calculating fine, but the Grand Totals on both the rows and columns are not working.

Does anyone know why? Or, even better, can anyone help me fix this?!

I've attached an example table with data and a pivot which gives a good summary of what I'm trying to do.

Thanks!