Kindly check the version of X and Cornerstone that is active on your site and make sure that both are updated. Here is a link for a details information on how to do an update:
Please remember to clear all caches when updating so that the code from the latest release is always in use. This will help you to avoid any potential errors or any unexpected display issue on the site.
In case the issue persists, please 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.