I'm a Marriott Titanium member, and I always call the Titanium number (I think it's the same reservations department, you just get through to them quicker, they recognize your status, and they might be better trained) to have my MR# added to my exchanges. I give them the reservation number, they add my MR# to it, and then I can see the reservation in my Marriott account.
The last time I did that, it didn't appear in my account, but I was still able to find the reservation with the reservation#. When I looked at the details, it showed the reservation# as my MR#. THEY had messed it up. I couldn't change it.
I called back and explained the problem to the next agent. They said they weren't sure they would be able to change it, but they tried and it worked.
If you don't get someone on the first call who knows what to do, hang up and call back.