Not sure if you need additional info on this, but I experienced it after being inv to join a group, then being removed cause they didn't want two hpallys. So, after that I wasn't able to waitlist for anything in SoO on my hpally, even after exiting the game and logging back in, and resetting all instances. I joined up with a guildy and confirmed that the game would let me in at the correct boss. Haven't tried yet on a dif toon.