Table view collapsing due to relationships issue.

I have the below model view in my Power Bi report.

the quiz summary has the unique quiz id's and has a many to one relationship with quiz activity, quiz attempts and eligible users.

The eligibleusers table has a many to one relationships with the custom userdetails table (userid column)

When I create a table view in desktop, containing the columns from quiz activity alone and add username column from the customuserdetails table, the view collapses (quiz activity has an username column as well). However when I take all the columns from quiz summary and add username from custom user details table, then the view is not collapsing. 

I am not able to build a relationship between customuserdetails table and quiz activity table as it saying it is ambiguous.

Can anyone let me know why this is happening?, all relationships are bi-directional only and there are no many to many relationships.