Gotta say this just cuz everyone else has clearly and completely answered the question so there is not a need to repeat those things... BUT....
no matter where we are "missionaries" for the Kingdom, for God's glory, there is always more work to do. So we, the leaders, post job descriptions and try to recruit people to fill in roles to make the work that we think God wants to happen, successful and completed according to his will. BUT as a leader, I can tell you, there are many many (many , many , many) times that we pray and think God is calling someone to a task and they say, " Wait til the children are older" or " I am just too busy now I have to say "no". or "I have too much on my plate" or some other career oriented conflict. I specifically have prayed and prayed about a few needs in the ministry here and I "know" that God has called some people to those tasks, but, they remain unfilled because LIFE just keeps rolling along and it is really hard to STOP and change routes. People get into a routine, they have a way they live, they like the way their life is going... or at least it is comfortable and predictable. People, maybe even Moses, (though I can't pretend to know what was going on in his head) tend to like the status quo even if it is not great. The alternative is change which most people are resistant to. (Come on Mose... pack up your bags, gather your family, quit your job, kiss Jethro good bye and head to Egypt) is quite a lifestyle change. So to truly grasp what makes answering God's call so difficult, for Moses and for the average Joe, we have to grasp the tremendous faith it takes to literally drop the nets, the fish, the boat and the family...right where they are.. and follow.