Custom Field Doesn’t Insert After Publishing

This is how it looks when I try to add a new post:

Screenshot

This is the code I use:

add_post_meta($post_id, 'Post Thumbnail', $post_thumb, true) or update_post_meta($post_id, 'Post Thumbnail', $post_thumb);
add_post_meta($post_id, 'Project URL', $url, true) or update_post_meta($post_id, 'Project URL', $url);
add_post_meta($post_id, 'Project Thumbnail', $thumb, true) or update_post_meta($post_id, 'Project Thumbnail', $thumb);

I complete the fields, hit update then publish and when the page is refreshed, the fields disappear. I have to go down to the dropdown select them and add them again to make it work. Any ideas?