Hide Featured Image box – isn’t working?

The context or the 3rd parameter of remove_meta_box() function for tags meta box and featured image meta box is side. And perfect hook to remove meta box is add_meta_boxes. So try the following code, hope it’ll work as expected.

function my_remove_meta_boxes() {
    if ( ! current_user_can( 'administrator' ) ) {
        remove_meta_box( 'tagsdiv-post_tag', 'post', 'side' );
        remove_meta_box( 'postimagediv', 'post', 'side' );
    }
}
add_action( 'add_meta_boxes', 'my_remove_meta_boxes' );