Hello, @Sreenivas,
I can see in my browser console there’s a JavaScript error triggered on product pages: https://postimg.cc/vckRbXRM
Variable products use JavaScript code to reload the variations, so errors like this one can prevent them from working correctly.
I checked the error details, and it looks like some of the default JavaScript libraries used in WordPress aren’t loaded on time. This can usually happen if there are some caching plugins that try to “optimize” JavaScript code by delaying it.
You can check and turn OFF the Options under XStore >> Theme Settings >> Speed Optimization. Try to check every option in that section one by one.
Otherwise, please try to disable all the third-party plugins (including cache plugins), that does not come bundled with XStore https://xstore.helpscoutdocs.com/article/36-included-plugins , clear the cache of the browser, and check then. Maybe one of the additional plugins is a reason for the problem.
If the issue still persists then please keep the plugins disabled and let us know I will debug in deep to find the cause of the issue and also your permission to debug your site which includes deactivating all the third-party plugins and on-site Customization as well.
Note: Please take a complete backup of your site first, before going to disabling the plugins to avoid any inconvenience.
Regards 8Themes Team.