Microsoft SQL Server 2019.Database Diagram not showing relationship and objects without dependencies

Hi Everyone. I have this situation.

After restore a database backup from another instance and generate Database Diagrams with apparently success, the diagrams are not showing any relationship between the tables. And when I see the dependencies for any table, there are no dependecies. The backup is from a commercial POS software that is been used for many business in my country.

What can cause relationships to not appear? 

I do that before (restore and generate database diagrams) without problems.

Best regards,

Francisco