Hi,
Normally, all the Joomla users should also be listed in the Customers menu of HikaShop. That is handled by the synchronization plugin of HikaShop when a user registers and an additional synchronization is done for all of them when HikaShop is installed or updated or when you click on the "check database" button of the HikaShop configuration.
So you should never not have a corresponding entry in the customers listing of HikaShop for a Joomla user.
It could happen if you had messed with the database by, for example, importing/exporting HikaShop from another website while the user accounts would not be migrated, or another extension changing things on the Joomla user account side.
Hard to say precisely.
Click on the "check database" button of the HikaShop configuration and you should have all the user accounts synchronized and it should then work for all the user accounts.