Session.getActiveSession() returns null although I'm connected

Session.getActiveSession() returns null although I'm connected

I use com.facebook.widget.UserSettingsFragment to manage login/logout to
Facebook in my app:
UserSettingsFragment userSettingsFragment = new UserSettingsFragment();
Session session = Session.getActiveSession();
if(session==null || session.isClosed()){
userSettingsFragment.setPublishPermissions(Arrays.asList("publish_actions"));
}
fragmentTransaction.replace(R.id.content_container, userSettingsFragment,
"userSettingsFragment");
In another Fragment (but same Activity), Before publishing on Facebook I'm
checking whether I'm logged using
Session session = Session.getActiveSession();
if (session != null && session.isOpened()){
publishOnFacebook(intent.getExtras());
}
When I start my app and try to publish, I always get a null Session
although I logged in before. And when I start UserSettingsFragment, it
shows me that I'm connected.
How can Session.getActiveSession(); be null if I'm actually connected?
Moreover, if I I start the Fragment where I do the publishing straight
after UserSettingsFragment, I get the correct Session, i.e.:
1 start the app
2 call `Session.getActiveSession()` in another `Fragment` returns `null`
3 start `UserSettingsFragment` shows that I'm log
but
1 start the app
2 start `UserSettingsFragment` shows that I'm log
3 call `Session.getActiveSession()` in another `Fragment` returns a session
Anybody can help?