This has happened to me quite a few times; sometimes when I play the Archon Priest strike, usually only on the nightfall or weekly heroic, after you have finished the first darkness zone where a hydra spawns. You enter a valley where a fallen ketch (giant ship) is floating nearby. You have to kill the enemies at the bottom of the valley, and are given the objective to summon your ghost in order to breach the door. After this, you have to survive and eliminate three waves of vex. What I have found happens to me and apparently to quite a few other people is that when surviving the waves of vex, it resets to another wave, or wants you to go and summon your ghost. Usually it only resets once or twice and goes away, but sometimes I have to summon my ghost over and over again and deal with multiple waves before it eventually works, or stops spawning waves completely, in which case I have to return to orbit. Anyone else experiencing this issue? If so, any solutions?