If user is logged in not working

Depends on a few things… just using that code probably won’t work if it doesn’t know where to echo it. For example the below will output at the start of the loop on the frontend of the site, assuming your theme is using the loop…

add_action( 'loop_start', 'message_when_logged_in' );
function message_when_logged_in() {
    if ( is_user_logged_in() ) {
        echo 'logged in';
    } else {
        echo 'not logged in';   
    }
}

If that doesn’t work try this since most all themes use wp_footer()

add_action( 'wp_footer', 'message_when_logged_in' );
function message_when_logged_in() {
    if ( is_user_logged_in() ) {
        echo 'logged in';
    } else {
        echo 'not logged in';   
    }
}