Adding values from different workbook
 

I have 2 workbooks; an Invoice Register (for all jobs and phases) and a
Project Cost Control workbook.
I need to search the entire Invoice Register finding and adding values based
on job name, phase code AND whether or not paid ("X"). I then need to add
all the paid invoices for that job and phase code and bring that number into
the Project Cost Control sheet for that phase and job. I have been using a
sumproduct formula that has worked but with intermittent errors. Now it
isn't working at all. I think perhaps the errors were caused by Blanks in
the Invoice register? Could that be correct? I didn't get an error but I got
wrong amounts in some places.

Anyhow, I'd like to construct the formula properly so that i can trust the
results.

I'm using Excel 2003.

Thanks,