Clear caches
Clear your caching plugin, host cache and browser cache before anything else — many “bugs” are stale files.
Check for plugin conflicts
Deactivate all plugins, then reactivate them one at a time until the problem returns.
Turn on debugging
On a staging copy, set WP_DEBUG and WP_DEBUG_LOG to true in wp-config.php and read wp-content/debug.log.
Contact support
Include your WordPress, PHP and theme versions, the steps to reproduce, and a screenshot.