I tried checking through the code inspector and there is no indication that the header is set to fixed upon scrolling. Kindly try to double check the setting in X > Theme Options > Header > Navbar Position and set it to Fixed Top,
Please also clear your site’s cache if you are using any caching plugin.
In case you have double checked that you have correctly set the fixed top header and it is still not working, kindly provide us with the admin details of your site so that we can check it. Please post the details in a Secure Note.
The option mentioned by @jade should work if X is not modified or installed properly. It looks like you have not installed X properly and/or have modified the parent theme. You don’t have X in your themes and the Cornerstone menu should not be there under the Admin Menu if X was installed correctly.
X should be installed directly under your themes folder. The common mistake is installing X in a sub-directory like wp-content/themes/x/x.
If you have altered the parent theme (which is not recommended), please revert your changes or delete X and reinstall a fresh copy. You can get a fresh copy in https://theme.co/apex/dashboard