Hello.
Its most probably that you are having some JavaScript error at that page. Please recheck your page for any JavaScript errors (usually for see what JavaScript error you have you need to press Ctrl {+} Shift {+} J for showing browser console) and try to fix them.
You can check for the most common issues on the FAQ page https://wpbookingcalendar.com/faq/ at the troubleshooting section.
You can also try to deactivate one by one all your active plugins at the WordPress > Plugins menu. Then retest it again.
If its not help, please deactivate your active theme and active the default WordPress theme. And then retest it again. May be there exist some conflict with some active plugin or actual theme and just need to find the reason of that issue.
Otherwise Please recheck your error.log in your server configuration, about any relative errors. If you can not find error.log file, please contact support of your hosting company about helping in finding this file.
Kind Regards.