I don't see how that's possible. The system bases itself on whether prices were found or not to display the add to cart button so if you see a price, then you should see the add to cart, even more so if you see it for other user groups as it means that it cannot come from a global setting like the "catalogue mode" option of the HikaShop which can remove all the add to cart buttons.
So that leads me to believe that it comes from a customization of the product view.
Do you have a folder template/YOUR_TEMPLATE/html/com_hikashop/product on your website and if you do, if you rename it via the FTP, does it solves the issue ?
If yes, then the issue indeed comes from a customization of the product view and you should contact either your template provider or the person who did the modifications for help.
If no, then please provide a backend access via our contact form with a link to this thread so that we can check your settings.
www.hikashop.com/support/contact-us.html