I am having this trouble too - only with select products in my store. Chatted with a guru today but they weren't able to help and passed it on to the 3rd party designer. It seems like a regular issue - Shopify, are you still working on this? I'm loosing sales as not every customer will contact us when they can't purchase. Help!
Same problem here with http://www.caseysbakery.com/casey-s-online-store.html . I made it in Adobe Muse. Can't find a rhyme or reason to why it does it sometimes and not others. Seems fine on some devices and not others.