When adding new files to a folder, nothing happens!
You should not expect the watched folder function to work immediately. I have not found any tech doc about it, but in my tests, it seems that in PSE2019, the search only begins after a restart of the organizer.
Where to find manual start of scanning folders for changes?
There has never been a function for manual scan for watched folders.
That's the reason why I don't consider it is useful at all for me.
After all, without watched folders, you simply import from the folder. Files already present in the catalog are skipped and new ones are imported. I don't like automatic background processes which take too much resources when you are editing or organizing. I prefer to choose when to start the import myself, very often when I am away from the computer. Since you can check the last import batch, it's even easy to cull photos you don't want to keep.
I have restarted the organizer several times - no effect, the folder does not scan
I am on Windows 10 and I don't trust the watched folders feature, which works or not... I have not found why and when.
The fact your watched folder is in an external drive may be relevant. Would watching a folder in your internal drive work?
There has always been issues in the past, but if the feature did not work for most users, the forum would be full of complaints, which is not the case. I am tempted to recommend posting in the feedback forum: