Before your next visit, you may want to go into
MDE and 'inactivate' all but the current magic bands you are going to use.
Like you I have about 20 cards and magic bands just for me. So when my MB wouldn't open the door, it was because the card I was using was deactivated. It happened especially with split stays. The CM is looking through all these cards trying to figure out which one goes with your room. Anyone of them should work, but the CM's (I think) try to deactivate all but one when they check you in. At least that's when they access your MDE.
You can than activate whichever color you want to use - old ones, new ones whatever, if you have your laptop (or maybe tablet or smartphone) with you.
I've found when I have more than one MB activated, sometimes one will get deactivated. I then just pop into MDE and reactivate it. It would be nice if they would give us a delete button, then we could toss older MB's and clear the stash.