Problem with custom field "variant child" in vm 3.0

Good afternoon,
I'm trying to bring stockaje on the sizes of the products, for this I am using the custom field of child variant. What happens is that only works the first time I select a variable. I leave an example of a product:

http://trevimoda.com/women-s-brands/coleccion/camisas/camisa-sisa-detail

In this case, the size "S" has 2 stocks, size "M" rated 4 stocks and "L" is 6. Well, if the "S" switch to the "M" I changed the stock level but if I return to the "S" or any other, no longer makes me change product or stocks and path of the product passes to be the selected size (ie, a non-existent route, if you can give a clue).

If I rename the "templates/gk_instyle /html/com_virtuemart/productdetails" directory to "templates/gk_instyle/html/com_virtuemart/productdetails-old" it's works properly, so I guess it is a template error.

Hope you can help me.
Greetings.

Pls anyone know anything about the problem is happening to me?

Thanks.