Wordpress custom filed not showing in list

[admin]

I have a post meta saved to the DataBase and I can see it in the DataBase and in the front end of wordpress.
But in the admin panel i can not see it in custom fields drop down menu.
like it is not there.
First I think the limit is on how much values to be displayed , And i change the value to 100 to be displayed.
And i still can not see the option in the drop down.

 add_filter( 'postmeta_form_limit', 'meta_limit_increase' );
   function meta_limit_increase( $limit ) {
    return 100;
 }