Once again, thanks Dan for this valuable advice. 'Finicky' and constant 'tweaking' and adjusting certainly apply. Temperature also plays a role because once the hub is warmed up it works better, and then, the next morning, it is uncooperative once again. I have previously dribbled light oil into the spindle hole, by laying the bike on its side, because I had the impression that the gears and bits were sticking and not moving as they should. So far that has not made much of a difference. I also suspected that some spring inside the hub was not doing its job, because often, after engaging a gear nothing would happen, and then, a while later, the selected gear ratio would finally engage. I accept your 'diagnosis' that perhaps parts are sticky and need to 'run in' with use. Will see what happens.