I thoroughly tested the plugin YITH WooCommerce Product Add-ons version 1.2.3 and I’m 100% sure it’s not working with the Xstore Theme. I turned off just the theme and the plugin works without fail, if I install that same plugin on anything else it works fine as well. I have an option heavy site that I’m getting ready to deploy and can’t until this theme is fixed and works with the YITH WooCommerce Product Add-ons plugin.
Could you describe me in more details what compatibility issues with this plugin did you face?
If you have problem with adding products to cart then go to Theme Options > E-Commerce > Single Product > Ajax Add to Cart > choose Off.