I tried to take a look at your site, but I’m only seeing a maintenance page:
https://cld.wthms.co/DNUMDi
It sounds like something is interfering with the variation switches – you can check the console to see if any errors are coming up. Here’s how to get started:
https://codex.www.ads-software.com/Using_Your_Browser_to_Diagnose_JavaScript_Errors
Alternatively – issues like this are often caused by either a conflict with your theme or with another plugin. The best way to determine if the issue is being caused by a theme and/or plugin is to temporarily switch your theme to Storefront (https://www.woocommerce.com/storefront/) and disable all plugins except for WooCommerce. A good way to do this is with a free plugin called Meks Quick Plugin Disabler (https://www.ads-software.com/plugins/meks-quick-plugin-disabler/), which will remember what plugins you currently have active so it’s easy to switch back.
If that resolves the issue, then slowly re-enable features until you find the one that’s causing the conflict.