Hi cphx. On post editing, have you checked under Screen Options at the top to see if “Featured Image” is selected?
I got the same problem and cannot see Featured Image under Screen options either anymore. Could it be after the upgrade to WP 4.3.1?
Most common causes I found online were:
1. A plugin conflict that is overriding the theme support for featured images. To see if this is the case you could deactivate all plugins and see if that fixes it. If so, reactivate the plugins one at a time until you find the one that causes the problem.
2. Some users reported that closing their browser (to clear the cache) and logging back on to their site fixed it.
3. If you have a Multisite install you need to go to Network Admin > Settings > Network Settings > Upload Settings and check the Media upload buttons (unchecked by default).
4. Try switching to a default WP theme like Twenty Fifteen and see if there’s any change.
