Well, I can see their frustration in not achieving the primary objective of the mission. Which wasn't achievable anyway because the whole mission was a hoax to get ahold of a Fallen but that's another story they are not aware of.
Ah, some players like being challenged by their GM and others like the GM tailoring scenarios so that they, the players, normally "win" the mission. In auto-cruise mode. Me, I think that only a few losses and partial successes make real victories meaningful at all. How else to build a sense of accomplishment?