WordPress won't go into maintenance mode

I have tried all the plugins just in case, made sure I logged out as admin, but my installation of WordPress won’t go into maintenance. The plugins say they are in maintenance, I log out go to the front end and see the website as normal, why would that be, how do these plugins work, do they use redirects, could the be effected by .htaccess if it says something else..what could it be…any help please