add_user_meta and update_user_meta working only sporadically

[admin]

I'm trying to display something to my wordpress site users depending on the number of times they have logged in. I've tried to accomplish this using user_meta and the wp_login hook.

add_action( 'wp_login', 'survey_login' );
function survey_login() {
    global $current_user;
    get_currentuserinfo();
    $login_count = get_user_meta($current_user->ID, 'login_count', true);

    if($login_count == "") { $login_count = 0; }

    update_user_meta($current_user->ID, "login_count", $login_count++ );

    if($login_count >= 5) {
        $_SESSION['csm_survey_login'] = true;
    }
}

This seems like it should work, but for some reason the user_meta key only gets added/updated about 1/20th of the time. I've been able to find no pattern to how or why.

I've tried simplifying the function to simply

add_user_meta($current_user->ID, 'login_count', 1);

Or

update_user_meta($current_user->ID, 'login_count', 1);

Both are giving me the same trouble.

<strong>Anyone know why update_user_meta or wp_login may only work a fraction of the time?</strong>