Session doesn't die with ALTER SYSTEM KILL SESSION

Summary:

I'm trying to kill a session with ALTER SYSTEM KILL SESSION '<SID>,<Serial#>'; as the Administrator user.

When execute the ALTER SYSTEM KILL SESSION, I see:

System KILL altered.

But when I query V$SESSION, the session status is still ACTIVE.

I've also tried

ALTER SYSTEM DISCONNECT SESSION '<SID>,<Serial#>' IMMEDIATE; as the Administrator user, but when I do, I get:

Error starting at line : 24 in command -

ALTER SYSTEM DISCONNECT SESSION '48333,52001' IMMEDIATE

Error report -

ORA-01031: insufficient privileges

01031. 00000 - "insufficient privileges"

*Cause:  An attempt was made to perform a database operation without

      the necessary privileges.