Hi,
It could be that the emails are rejected by your email server for their content.
For example, if someone is called Mr Viagra and that your email server has a spam filter which checks the content of the emails before sending them, it could reject the email without notifying HikaShop because viagra is on the blacklist of the spam filter and you wouldn't know it and you would never receive the email.
Normally, in that case, the log of your email server would contain the reason of the rejection. In that case, the best is to ask your hosting company (which manage your email server I suppose) that they look at their email server log. Give them as much data as possible on the email itself (to which address email it was sent, with what subject, etc) and on the way you configured your joomla and hikashop email settings and they should be able to tell you what's going on.