Help with Nephthytis plant propagation?

Hi, I have a small nephthytis plant that is doing all right but I have tried to propagate it as people have told me by taking a leaf stem and placing it in water. Either nothing happens or the cutting turns yellow and dies. The plant and leaves apparently don't like bright light and that might be the problem. Should I check for roots since I've never seen them on my cuttings. What am I doing wrong?