There is one thing missionaries (really, anyone) have to be able to do to succeed:
They must be able to stop.
They must be able to say, “I’m not going to do _x_ any more.”
They must be willing to try something, be able to measure what they do, and commit to evaluating and learning.
They have to be willing to stop doing something that doesn’t work, and try something else instead. They have to be able to change.
Missionaries had to do that at the very beginning of their career. They had to stop doing what they were doing in order to start on the mission path. (Same for just about anyone – entrepreneurs, artists, marriages and the like.)
No one ever found their success right off. Success requires starting, stopping, and starting something else.
If someone’s not willing to say “no more” to what they’re doing right now, they will never be able to say “yes” to a call to missions.
In whatever you are doing, are you (1) measuring the results and (2) willing to stop doing it if it doesn’t measure up to a certain benchmark?