First of all, really sorry for the delayed response. Support crew is on summer vacation thus it causes some delay on replying inquiries.
I left alone and trying to catch up. But, starting from August 15, vocations will be over and everything will be get back to normal like before. Normally our support-get-back time is within 2 business days.
Maurizio, the problem you are experience is happening due to CSS Conflict (Conflict between CleanMart and your Joomla Template).
The solution is pretty straight forward:
FOR THOSE WHO EXPERIENCE THE SAME PROBLEM CAN APPLY THE SAME SOLUTION
1- Open this CSS file:
/templates/YOUR_TEMPLATE/html/com_virtuemart/assets/css/flexible.css
2- Scroll to the bottom and include this CSS rule:
.FlexibleCSS3ButtonQuantity{ box-sizing: content-box;}
That's it. This will fix the problem. The quantity box will be displayed upon rollovering the add to cart button.
Regards.