No wonder the church can't grow. There's a bottleneck. The secret to motivating people to serve over an extended period is to give them ownership of the ministries they operate. Allow the people leading each ministry to make their own decisions without a lot of interference from your board. When you give authority with responsibility you'll be amazed at people's creativity.
People respond to responsibility.
Expect the best of your people, and trust them with ministry. Give them a challenge, give them control, and give them the credit. Honestly, I don't like the term lay minister because it can imply second-class citizenship and incompetence. In a biblical church there are only ministers. We should treat those who serve without pay with the same respect as those who are paid for their services.