networks-for-wordpress

Dear Sir, only the site(s) on the root/first network can successfully logout. For any other networks and their associated sites, when we try to log out we get the WordPress Failure Notice prompt asking if we really want to log out. If we confirm we want to log out it will take us back to the login screen saying you are now logged out, but it never actually logs out of the user. Going back to /wp-admin takes you right in. I then manually clear all the cookie from the browser. When the cookie are cleared, I see that the sessions are actually logged out now. It means that in the network and associated sites, cookies are not getting cleared upon logout. Can you please help me to solve this issue. Thanks a regards