I can't decrease cuantity products in the order

Good afternoon:

I was very happy with the plugin. But I found one problem.

I created one rule for one product (tag). Minimun 2, step 2. Everything was ok. The quantity in the producto page is ok. I can increase properly from 2 to 4, to 6,… but, when done this ¡I can’t decrease!

The link for the product with I’m testing (Spanish): http://138.128.171.138/~silleria4ragones/shop/silla-enjoy-plegable/

May be the reason is the incompatibility with the theme?

The plugin is perfect, congratulations, this is the only problem I have. Thanks for your quick response

https://wordpress.org/plugins/quantities-and-units-for-woocommerce/