Out of stock, but there are 20 items

-- url of the page with the problem -- : www.trazionemoto.it/shop/ricambi-e-acces...otul-7100-10w40.html
-- HikaShop version -- : 2.3.2
-- Joomla version -- : 3.3.3
-- PHP version -- : 5.4.27
-- Browser(s) name and version -- : firefox
-- Error-message(debug-mod must be tuned on) -- : Product "Out of Stock", but there are 20 items

Hi,
I uploaded an article with the amount of product 20, but it gives me the message "esaurito (out of stock).
If instead the public a product, with a variant, there are no problems.

Look at:
www.trazionemoto.it/shop/ricambi-e-acces...otul-7100-10w40.html

thanks