Hi,
I'm curious to see what happens if your Record Selection Formula was only:
Date ({CallLog.RecvdDate}) in Lastfullmonth;
If you created a Group on Ticket and a group on CallStatus and a last group on AssignStatus.
This should break it down and in the Group for AssignStatus, what values do you see? I wonder if there are NULLs which your formula may have to handle.
If the CallStatus and AssignStatus are in their proper place, now go back to your Record Selection Formula and try filtering out what you don't need.
Good luck,
Brian