Have you ever been involved in a project from beginning to end? It's very satisfying to see something you started and have been involved with come to completion. But, it is equally as frustrating for us to begin something, being involved at the beginning, but never getting to see it through to completion.
Paul experienced this. In fact, he wrote to the church at Corinth about this very thing. He talks about planting seeds, and Apollos watering them. But he is quick to point out in the same verse that it was God, not either of these men, who gave the increase.
Are you planting seeds in someone's life? Maybe you find yourself as the gardener instead, being used to water the seeds someone else has planted. Regardless of how God is using you in someone's life, it is God Himself who gives the increase.