get_post_meta in admin columns

I tried to add a custom column using get_post_meta but its not displaying?

class Prefix_Custom_Admin_Columns extends MB_Admin_Columns_Post {
	public function columns($columns) {
		$columns = parent::columns($columns);
		/**
		 * Add more column in a specific position
		 *
		 * @param string $position New column position. Empty to not specify the position. Could be 'before', 'after' or 'replace'
		 * @param string $target The target column. Used with combination with $position
		 */
		$this->add($columns, 'col_category', 'Category', 'after', 'title');
		$this->add($columns, 'col_featured_image', 'Image', 'after', 'col_category');
		return $columns;
	}
	public function show($column, $post_id){
		switch($column){
			case 'col_featured_image':
				// This Works fine
				the_post_thumbnail(array(70,70));
				// This doesn't work
				// echo get_the_post_thumbnail(array(70,70))
			break;
			case 'col_category':
				// Not Displaying
				echo get_post_meta($post_id, 'brn_category', true);
			break;
		}
	}
}

even i tried to echo get_the_post_thumbnail() is not displaying too