In Firefox press F12 click on Network tab
then click on trash to delete history 
select XHR on sub tab 
Then click on a combination you should see a POST action, click on the line and then clic Response tab 
you should see the json result to refresh part of the product page. If you don't see any error or empty response. You can switch Prestashop to debug mode on start the process again an see error message in Response. That could help you to find the origin of the trouble.
In Firefox press F12 click on Network tab
then click on trash to delete history select XHR on sub tab Then click on a combination you should see a POST action, click on the line and then clic Response tab you should see the json result to refresh part of the product page. If you don't see any error or empty response. You can have switch de Prestashop debug mode on start the process again an see error message in Response. That could help you to find the origin of the trouble.
