Hi,
Which "registration email" are you talking about ?
The confirmation email which is sent to the customer when he registers and he needs to confirm his account ? Or the notification sent to the admin for newly registered users ?
Also, on which pages do you have the issue ? The Joomla registration page ? The HikaShop registration page ? The checkout registration ? Two of them ? All of them ?
And finally, if it's an email that is sent by HikaShop, then you should see it in Customers>Emails history. Do you have it there ?