No "add to cart button" with IE

Hello,
I’ve had customer complaints about not being able to make purchases through the website for months. This problem only occurs with Internet Explorer, and everything seems to work fine with Chrome, Safari, Firefox, etc. I do not know what variations of IE vs specific OS are not working. I am running Windows 8 and I can not duplicate the problem. I have IE10 installed (because it came with the computer!) I’ve used the F12 developer tool to try it in IE10, 10 Compatible, 9, 8, & 7 and I have been able to see and use the “add to cart” button every time. The complaint I get from people is simply that there is no “add to cart” button, and I have checked to make sure they are selecting size and colour (where applicable). I redirect them to use a decent browser, when possible, and they encounter no problems. Is there something I can do to correct this error, aside from admonishing people for their choice of browser? ??

Thanks

url: https://www.goodshelf.co.uk