I am using hello_minty blue theme on Magento 18.104.22.168 version. When click “Add to Compare” on a product, the success message “The product *** has been added to comparison list.” shows up, but the “Compare Products” section on the left side bar still only shows “You have no items to compare.”.
I have made sure the attributes like “description”, “short description”, “sku”, etc. are comparable on frontend.
As pointed out by the other thread, all products need to be included by the default category to make it work. I guess your demo site has the same configuration. It’s a weird requirement, but I can go with it.