Hi, if in woocommerce, the variation stock is set as 0, then your plugin needs to show a crossed box isn’t it?Because that’s what goes wrong sometimes. In the woocommerce stock it shows 0 stock, but your plugin shows it as available, and when you add to cart you get a message: product stock is 0.
Could you record me reproducing your issue using a tool like this? https://www.useloom.com
It helps me to quickly figure out the issue and squash it with immediately.