Water lettuce can be fickle. I buy some each spring. Some years, it doesn't last long. Other years, it has covered the entire pond by fall! Here's a photo from 2009 of the pond completely covered:http://www.fishpondinfo.com/photos/ponds/bigpond/0827091.jpg
Water lettuce does best in an established pond with enough fish to provide nutrition, not full sun but close to it (it doesn't like extremes in temperature), a warm summer, and low water movement. You can try to supplement feeding with liquid plant supplements, especially ones with potassium in them which is often a limiting nutrient in ponds.