Our Product Accessories typical theme installation does not always integrate with each theme out of the box. There is also a possibility of another app conflicting with ours.
In the event that your add to cart button isn't showing or disappears when clicking certain things, we recommend removing the widget snippet from your product.liquid. This will temporarily disable the product accessories across the site.
In the meantime feel free to open a support ticket with us so we can take a look at whats going on.