Sometimes we have to wait for a long time before what God has promised comes true. It's not that God delays things on purpose; sometimes the people whom God has invited to join the work just don't show up.
We might wonder why God does not invite only the people whom God knows will show up? Because God is patient and wants everyone to have a change to join the work, whether they choose to do so, or not. Knowing how many times we have refused ourselves, God's patience is a great source of comfort for all of us.
It is easy to become bitter when things don't work out; when we don't know why we have to wait, when we are ready to do the work but nothing seems to work out. But when we choose not to walk away from the work when we meet unexpected delays, patience becomes our salvation.
Patience - the ability to control ourselves when frustrated - produces endurance in us, endurance produces godliness.
For this very reason, make every effort to add to your faith goodness; and to goodness, knowledge; and to knowledge, self-control; and to self-control, perseverance; and to perseverance, godliness; and to godliness, brotherly kindness; and to brotherly kindness, love. For if you possess these qualities in increasing measure, they will keep you from being ineffective and unproductive in your knowledge of our Lord Jesus Christ (2 Peter 1:5-8 NIV).
Instead of becoming bitter and frustrated, we should find joy in the other things God has given us while we wait patiently for God's promises to be fulfilled, for there is more to life than work, even when we work for God.