Please make sure that you are using an updated version of both X and Cornerstone as it will cause some incompatibility use just like Cornerstone not loading if either of them is not updated.
In case you are using an updated version of both, try testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
If you are still having an issue, please provide us with the admin access to your site in a Secure Note: