Help with Query Builder - Problem Reports NOT linked to a Change Request

Hi All,

I'm trying to use Query Builder/Report Builder to find all Problem Reports in the System that are not linked to a Change Request.  I thought I was close using a Join and Formalized By which would give me the Problem Reports that do have a linked Change Request but I ended up getting some erroneous exclusions.  (Problem Reports that were not included in the returns using the Join with Change Request [WTChangeRequest2] and fomralized by).

Beyond that, anyone have any guidance on how to better understand and use this capability?

Thanks!