I am running into this problem also. I had all my ADR's booked for a 10 day trip in June with just my daugher and myself and have since added her cousin to the trip. I was only able to update one ADR to reflect this change (Coral Reef), some of the others had nothing available at all on the days I wanted, some at very late times.
Talked with CM who was checking reservations. She couldn't guarantee anything but did say I should turn up for reservation times and see if they could accomodate me. She did say I would have a much better chance of my reservations were for first eating times for that meal (i.e we have 8:05a, at Crystal Palace for breakfast and 5:00pm for dinner at Chef Mickeys).
I don't feel comfortable doing this, I don't want to mess up someone elses reservations, so I keep checking each day. I am also going to see if the conceriage at BC can help me once we check in. If not, I will probably cancel all but the two early ones.