Had something to do with their bubble... but was PvPing on Alderaan and there was a Sorcerer in the middle with her bubble up and I couldn't attack her, neither could anyone else. The only thing that worked were the AOE attacks. Just a heads up... this is 1.3
She had a huge amount of buffs. Are there any buffs which make you unable to be targeted? I think the Jedi have one which is similar to this, however I do not have a sorc.