Here's a wrinkle that finally got mine to work (and credit the reservations person who spoke the least English to come up with it).
My reservation was through Vacationoutlet.com, and since they're also a
travel agent that's the radio button option I thought I was supposed to use - it was the only one that seemed to make sense. Wrong.
If you have a long hotel confirmation number and it isn't working on it's own through mydisneyreservation.com, select the option for "through a group or convention" and see if it works. That's what brought up mine, and since I'm neither part of a group nor in a convention I would never have selected it on my own.