OP here. I retract my earlier conjecture that this was fixed by the new software; in fact nothing has changed. It would be nice if the simple approach described by others - just hold down the button - worked in all cases, but that's not my experience. There are lots of permutations and combinations of factors: Is the car on or not (whatever "on" really means in a vehicle without an engine), button held down or just pressed, handles out or in; power flowing through the cable or not, battery reporting full charge or not; light ring solid green, slow flashing green, fast flashing green or dark when approaching it. Too many cases to explore fully, at least not in an unheated garage in January.
I often have occasion to put a leash on a family dog. Sometimes he holds his head still to accept it. Sometimes he squirms around, and it takes me several tries, even though he really wants to go out. Taking the Tesla off its leash feels a lot like putting the dog on his. Good dog. Good car. Just a little mischievous sometimes.