Some people are just looking for quick and easy money and there is nothing you can say to them to get them to work.
With others, perhaps the timing wasn't right or they didn't understand. Send everyone weekly Team Mail. Something you say might motivate them.
Gery says we spend too much time worrying about our inactive team members. Work with the workers and keep adding more people to your team. Sooner or later you will find at least five active people.