@ptillisch
thank you but this doesn't work.
I have plugged in the nano via a switched USB HUB to avoid taking it out and it from the usb. Switching the nano off and on just before uploading is the only thing that consistently work, so it must be a issue with the board reset(?)... All the three nano boards I bought act in the same way.
But note that pressing reset on the board itself does not help with the sketch uploading. the reset button works fine. it resets the board. maybe it's a timing issue.... at this point the short term solution is clearly to press the usb hub switch just before uploading and it will work, but it would be nice to understand what is causing this issue and how to resolve it.