Hi,
The question is: Are the orders status confirmed ?
If they aren't, it means that you have an issue with the payment notification, and I invite you to review that documentation:
www.hikashop.com/en/support/documentatio...or.html#notification
If they are, then it means that the problem is the the sending of the email itself. If you receive the order admin notification, then, it can't be an issue with the configuration of the email settings of the joomla configuration or the hikashop configuration. So all that's left is the order status notification which is sent to the user after the order is confirmed. Maybe you modified it or disabled it via the menu System>Emails ?