If you need to manually enable the session globally, use this in your functions.php (I included a line for manually setting a session variable as an example, not required):
add_action('init', 'session_manager');
function session_manager() {
if (!session_id()) {
session_start();
}
$_SESSION['foo'] = 'bar';
}
and if you wanted to manually clear the session on an event (like logging out):
add_action('wp_logout', 'session_logout');
function session_logout() {
session_destroy();
}