Well, the pattern data from each obelisk can change each time you leave the instance, so perhaps the ancient items required to unlock the obelisk changes as well? If so, you'd never really be able to do comprehensive testing, you'd just have to brute force it until you finally get all 21.
This sounds like really bad design - you have to re log to scan the obelisks again. If each time you relog the combination resets and randomises it's not science, it's chance.