What could be preventing items from getting added to cart?
If items are not getting added to the cart after pressing checkout, or if incorrect items are getting added to cart, this may be caused by several issues but the main reason is due to a cache plugin, therefore you will need to configure the cache plugin so it does not cache the Smart Online Order pages (store page, cart page, and checkout page), in this article we will explain how to configure your cache plugin from caching these pages. You can also contact the plugin support for the various cache plugins for more information and support.
W3 Total Cache
From your WordPress dashboard go to performance then Page Cache, you will found a section named : ‘Never cache the following pages:’, insert the url of your pages without website name for example your store page is www.example.com/store, insert only store, (insert each page per line).