Yes, Missions can fill up quickly depending on the number of spots available and the popularity of the product. Some Missions could have just a few hundred spots while others have many thousands--so we're careful about the number of members we invite to a Mission.
On occasions when Missions fill up very quickly, you could be taking the survey as the Mission fills. This means that when you started the survey, there were spots available but they all filled up before you made it to the end. If you receive a Mission invite, act fast to avoid disappointment. Your spot is not guaranteed until you accept that Mission!