Hi,
It's probably because you have special characters in the category names, that you didn't configure any alias (without special characters), that you have the unicode slug option of the Joomla configuration activated, and that you have the "remove products and categories ids from URLs" option activated in the HikaShop configuration.
Because of all that, HikaShop cannot know what category to display when you click on a category link and that the alias in the URL doesn't match with any category name exactly. Turn off the "remove products and categories ids from URLs" option in the HikaShop configuration and that should help.