How to drag/fill a column with a 3D formula

Thanks to JBeauclaire here http://www.excelforum.com/excel-form...le-sheets.html I have the right formula, but can't fill the column.

In my monthly sheets columns B and C have Qty and UnitPrice. My annual summary sheet has the total Qty in B3, and I have used his method in C3 to find the average Unit Price for the year:

=SUMPRODUCT(N(INDIRECT("'"&Months&"'!B3")),N(INDIRECT("'"&Months&"'!C3")))/B3

My problem is how to replicate this formula down column C for all products. When dragged the row number does not increment - how can I do that?

Yes, thanks