the cart is update with quantity and the product, but when you going to see in another pc (or browser) you look the same quantity of the same item is updated!!!
finally, to give solution, I left everything the same as in gktemplate productdetails> default.php and I import from joomla only the track to submit the form add to cart.
So it's solved, right ? Recently I had a similar issue and had to override files of joomla and template even if it was in the latest version according to information in joomla.