Create a report on a field not being populated

I would like to create a report on a specific field used on a portfolio level custom form. Users are accountable for adding the custom form to the portfolio and completing it. I have a report for those portfolios that do not have the form attached, I now need to find out where the form has been attached, but not completed - by identifying one or two specific mandatory fields will determine this.

I have tried a Journal Entry report, in Filters selected the Portfolio>>[field] is Blank, which shows me no data, when I know there are empty fields somewhere!!

Thanks loads.