If you are using Worldpay payment plugin, you will need to make some changes in the settings of your Worldpay account. The steps are:

  1. Login to merchant account at worldpay, edit the installation, in the Payment Response URL textbox, enter YOUR_SITE_URL/index.php?option=com_osmembership&task=payment_confirm&payment_method=os_worldpay&notify=1, for example,
  2. Check on Payment Response enabled checkbox