WooCommerce Subscriptions 1.4 disabled some features on larger sites (i.e. sites with more than 2,500 subscriptions). This plugin can be used to enable these features if you are confident your server has enough resources to allow them to work correctly.

wcs-is-not-large-site.php

<?php
/**
 * Plugin Name: WooCommerce Subscriptions Enable Search
 * Description: Enable the subscription search and sort by next payment date features on large sites. <a href="http://docs.woothemes.com/document/subscriptions/faq/#section-60">Learn More</a>.
 * Author: Brent Shepherd
 * Version: 1.0
 * License: GPL v2
 */

add_filter( 'woocommerce_subscriptions_is_large_site', '__return_false' );