I'm using a plugin called Co-Authors Plus for managing guest authors in my website. However, when creating a new guest author, the Featured Image metabox (postimagediv) doesn't appears. I'm almost sure that it's not a problem with this specific plugin, as I've tried it in other Wordpress installation and the metabox did appear. I've already checked the Screen Options and it's not there. Also, when adding a normal post the Featured Image metabox do appear.
This is a screenshot from the non-working installation:
And this is a screenshot from the working installation:
Do you know what can be keeping this metabox from appearing? Maybe file permissions?