There's a commonality here that most people are unwilling or unable to admit. There is no technical reason why some people (DVC and TA guests aside) should be able to reserve their 90+10 and others cannot. Those who cannot either originally booked under the old CRO system and/or are punching in the wrong phone number associated with the reservation, or they are not logged in when they go to make their ADR reservations, which is why the system doesn't recognize their resort reservation. I realize no one will admit to this, and everyone will say the information input on their end is 100% accurate, but computer systems (including Disney's, yes I know) don't have glitches that way. If it didn't work for some people, it shouldn't work for all people, and vice versa.
In any event, for every person who has had a problem booking 90+10, I have heard of equally as many success stories. Maybe those who are having trouble gave the incorrect info when they originally booked their resort reservations, or maybe they are inputting the information incorrectly now. Either way, the info has to match on both ends and it's not. My belief is it has something to do with your phone number. That seems to be the consensus amongst all complaints.