Grey button shows when a product is out of stock

Hi!
great theme, we’re very happy. Thank you very much!!

I have a little issue with out of stock products. In those products, a grey empty button appears in the catalogue view (screen capture)
If it suppose to show an ‘out of stock’ message, i’d love to get it to work;
if it is an error, i’d love to get rid of it;
but is annoying the way it is now.

Do you have any idea how to fix it? I put an ‘out of stock’ label meanwhile, but i really need to fix that button.

Thanks in advance!
Mica

Here you can see out of stock productos in the site.