Reporting on Advanced Custom Objects

Hi,

We have created a report form Advanced Custom Obects from Reporting section. There is this report that is based on Opportunity and Competitive Bid (Custom Object) Objects, we have a filed on Competitive Bid called 'Action', we are having issues with the the condition where we want to display all the opportunites where the 'Action' field is NULL. Whenever I put that condition in the report come up blank with no data. I am not sure how to achieve this....I know in order to get data between two objects you have to create a relationship and I have done that by using