Just fixed the add to cart missing button issue I was having. I was told that plugins are a common factor for this type of problem, so I deactivated them all (except woocommerce) and switched themes to 2011 (default wordpress theme). I activated them 1 by 1 to narrow down which plugin was causing the trouble. Not sure how, but after reactivating them all, my add to cart button appears just fine and it looks as if the issue resolved itself. So switching out of my theme, deactivating plugins (except woocommerce!!!), reactivating my theme and reactivating my plugins fixed it. Not sure if this will work for anyone else, but I thought id offer the example seeing as how this problem is stupid annoying. And everyone remember that you have to select an option (size, color, ect) before the add to cart button will appear!