Just noticed that you haven't gotten an answer yet. As I read your question, I'm not sure if there is enough information to really understand what is going on. I understand it would be difficult or impossible to post a packaged workbook in this case, but one thing that might help is at least a screenshot or two to see what fields are in play. Maybe also a screenshot or two of your data connection properties to see what's going on there.
I came across this old post when I was looking for "many-to-many" relationships in Tableau.
The "level of details" explained by Jonathan helped me achieve what I wanted to do.
If others like me will stumble upon here, I invite them to read the following: