Could you tell us which database you’re using and what driver you are using to connect to it?
Do you see the Primary and Foreign Key icons decorating the appropriate columns in the Table Nodes (in the ER Diagram)?
If not, can you check if these icons are visible when you expand a table in the DB Browser view / Does the Foreign Key tab in the Table Info view contain the correct information for your tables?
The most common reason for missing relationships is missing FK information.
Best,
Brian.