How can I get my older children to visit me more often?

I have 6 kids and 2 of them have already gone off to college. My oldest daughter moved all the way to California and my other moved all the way to a college in South Carolina at the age of 16. Even when they have breaks, they go off to other places and seem to forget about me. There are no financial issues at all. It just seems they have no will to visit me. How can I get them to come back and see me once in a while?