At least, as of the initial release, not officially.
If something got messed up and you deleted face regions thinking Lightroom CC / 6 would re-index them, you already found out that this doesn’t work. And Adobe provides you with no official way to restart face detection on an image, so this kinda leaves you in the lurch.
Nevertheless, there are two tricks that might help.
This first trick is the easiest, but only works sometimes. It’s not an official fix, so who knows? Adobe may see this as a bug and “fix” it sometime in the future, but for now it’s worth giving a try. Images that you rotate and have not had any confirming action run against them will re-run face detection. So if you need to re-index a batch of faces, select the images, rotate them, then rotate them back. You may need to wait for them to re-index after the first rotation before you can rotate them back. This is a “YMMV” (your mileage may vary) trick, but the upside is that it’s hard to do any real damage by trying it out.
The next trick I’m not going to give you full instructions on, because it involves direct editing of your Lightroom Catalog. This has the potential to ruin your life, and corrupt the catalog, so assume you’ve been given all the dire warnings about backing up, not trying this at home, etc.
A few enterprising users have opened the lightroom catalog with a MySQL Light client (that’s what Lightroom’s catalog is) and clearing out all entries from the table Adobe_libraryImageFaceProcessHistory. Be sure Lightroom is not running when you try this. If you make this work, let us know your process in the comments below.
If you found this article helpful, please follow on your favorite social media system and reshare!
[Edited for clarity.]