Multiple security roles not working properly

Hi Folks, 

In my Online MS CRM we have 20 security roles and most of the user have multiple roles assigned to them .

In one custom entity I want to give only read access to all while system admin can have all persmission .

But some user reported that they cant see that Custom entity they are seeing (****).

Can I know what might be the issue ?

Also do multiple roles works as union of access roles(create , write, delete) or intersection of roles.

Kindly suggest.