RLS - Need Help

I have a set of tables:

User table:

UserID       USerName

======= ========

   A               AAA

   B               BBB

   C               CCC 

Region table:

Region_ID     Region_Name

======= ==========  

     1                Reg1

     2                Reg2

     3                Reg3

     4                Reg4

Access_Details table:

Region_ID    User_ID

======= ========

      1                   A

      1                   B

      2                   B

Details table:

Region_ID   Date   Status Flag

======= ======== ===

     1                xyz         1

     2                abc         1

     1                abc         1

I need to limit access based on the User_ID. I tried creating a role on User table by specifying [User_ID]=UserName()

When i used "View as Role" and specified User_ID as A, nothing gets filtered.