Hello @PR,
In order to resolve your issue, please navigate to XStore > Theme Options > Woocommerce (Shop) > Single product Builder > Add to Cart & quantity > Stretch Add To Cart Button > Turn ON and save settings.
For a better reference, please see the image provided at the following link: https://postimg.cc/2qQsNGz3
We hope this helps.
Best Regards,
8Theme’s Team