As chikki mentioned try to use the click of link, else u can also use the webelement. I would like to know does this Add to cart weblement/link has any unique property that is associated with the product that is displayed above the button.
If u want to buy for example some ES2.. then does the Add to cart beneath has some reference to this product ES2..
The screenshot on spying needs to be detailed, so post the screenshot which can show more properties
I think as Eagles mentioned in here, we need 2 have a look at the HTML for the hierarchy. If the Add to cart is a directly under the item or does there exist a different hierarchy. We need to use DOM as there should be a unique selection instead of a random number selection.