CI' session question

[eluser]chmod[/eluser]
If i enable the item sess_use_database,set

Code:

$config['sess_use_database']        = TRUE;

then CI will auot save user's session data in ci_session table.
If I set the customer's session data , the data will be store in this table.
Like this:

Code:

$this->session->set_userdata('islogin' => TRUE)

I can get the customer's data:

Code:

$this->session->userdata('islogin');

but in another controller:
I can't get the data,why?