Hi,
Site B is using a pretty dated HikaShop version, obviously, and possibly an older Joomla version as well, so it works there, but you can't really compare with site A.
Anyhow, there are other threads on the issue, and here is one, for example:
www.hikashop.com/forum/checkout/878989-4...-adding-to-cart.html
You see that the issue was taken care of by a "hot fix" since your HikaShop 2.5.0 build. So, download the very latest & greatest, install it over your current one, and the issue should be fixed.