thanks for your reply (and the additional pm, which I just came today to see as I am quite busy in this time).
Will try to get an answer via the recommended way within the next couple of days esp. as it can not be an issue of the pictures, as all (!) of the pictures and portraits are not detected. Different photos, different portraits of different people. Some with even more then one picture. Strange...
The problem may be that there needs to be at least 5 instances of the same face in order for it to show up in the People section. If there are less than 5 pictures of any single face, it will not appear in the list.
The Lightroom desktop app is more helpful as you can look at the Keywords tab of an individual image that has faces in it, and on that tab will be entries for each of the identified faces, with the option to name them. Once you have named a face, it WILL then appear in the People section even if there are less than 5 instances of it.