Sun Coleus are grown from cuttings because they produce an exact clone of the mother plant. When you grow them from seeds, they will all be different. A few might resemble the mother plant, but they're never exactly the same.
I've grown coleus from the seeds off my plants for several years with spectacular results. They're very easy to grow. The seeds need light to germinate, so just sprinkle them lightly over the surface of your soil and water them in. You should start seeing them sprout in 7-10 days. It's really interesting to see all the different colors and leaf shapes that you can get from just a few seeds.
Did your bed have different varieties of sun coleus in it, or a mass planting of just one kind?