@dado023 It's not a problem of HikaShop. You'll have the same issue with any ecommerce solution for Joomla. The issue is the caching system of Joomla which isn't fine grained enough. There exist third party caching extensions for Joomla which you can use instead and which won't have such issues. Basically, if Joomla caches the whole HTML of all your pages, when the user adds something on his cart, he won't see the new cart because the page display is still picked up from the cache. What's even worst is that the cache is the same for everyone on the website so you would have situations where on some pages you would see the cart of other users.
In short, carts should not be cached as their role imply that they are dynamic.
Joomla is dynamic yes, but in another sense. When you change a joomla article text and save, joomla can flush the cache but everyone sees the same article on the frontend at all time and so in that respect, the joomla content is static.
@ceciliasp If you use protostar, there is no caching option in the templates so no need to worry about it.
Did you turn off the caching option of the joomla configuration and the system caching plugin of Joomla ?