I’ve been searching for the answer to this question for the past week and I finally fixed my problem, maybe this can help you guys.
In the admin-functions.php (in your wp-admin) folder find this:
function return_categories_list($parent = 0) {
global $wpdb;
return $wpdb->get_col(“SELECT cat_ID FROM $wpdb->categories WHERE category_parent = $parent ORDER BY category_count DESC LIMIT 100”);
See where it says LIMIT 100? I think that limits the number of categories shown to 100, I changed it to 1000 and all my categories appear now in my “Write Post” page. Hope this helps someone!