It won't work with certain images

From FAQ:

Can I use this plugin for setting featured image using some image not attached to the post?
No. This plugin uses only standard WordPress functions to set the featured image. And using this standard (and friendly) method WordPress simply has not any knowing about images not attached to the post.

Images must be attached to the post (uploaded from edit post screen for that post).

Thanks for your reply!

I read the FAQ and searched the forum before posting. The paragraph you quote doesn’t give an explanation. All the images I’m talking about are attached to the post the same way, but still, some get converted to featured image, some don’t. I’m not uploading them though from the edit post screen, I choose them from the media library, but my method should be just as good. On my other site your plugin worked with all my old posts without touching anything.

I’m not uploading them though from the edit post screen, I choose them from the media library, but my method should be just as good.

Like I said, plugin only works when the image was uploaded from the same post add/edit screen where you want the image to become featured image.

That it’s not a plugin’s fault, that is due to the way WordPress handles the relation between images you upload and the post. The only way for the plugin to set the feature image for you is to check wich is the first image you uploaded to the post. If you pick the image from media library (uploaded from other post screen), that image is not attached/related to the post by WordPress (because it already has a relation with the post where it was uploaded). So there’s no data for the plugin to work.

From plugin readme:

…it automatically sets the featured image to the first uploaded image for that post…

I can say it more times but not more clear.

Thank you, Samuel! You helped me a lot to understand how it works.

You are only partly right though. It turns out that you can insert an image from the media library and the plugin still works as long as the image is not yet attached to another post.

In my case, I uploaded a bunch of images in bulk using Media>Add New, so they didn’t get automatically attached to any post. That way I could insert them into new posts and your plugin worked. I just didn’t realize that when I trashed a post, its featured image remained attached to the post in the trash, so the plugin couldn’t work. I had to permanently delete the posts from the trash to make those images unattached and so the plugin work again.

Maybe it would be a good idea to update the FAQ accordingly.

Thanks for the great plugin!