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.
If you still will have this issue, please send the link to the page with booking form, where you are having an issue, we will check it then.