There seems to be a bug in product variations dropdown in the front-end of Jigoshop. When select a variation in the dropdown everythings works until you select "Choose an option..." a second time. This action disables all of the variations. The only way to make it work again is to refresh the current page in order to reset the form.
I've managed to fix this behavior by editing the assets/js/script.js and change the line 249