Lettuce is a spring crop, and produces more and tastes better when grown in cooler temperatures.  During the summer, keeping lettuce in a cooler spot in your home will produce better results.  And lettuce loves cold feet!  Some AeroGardeners put a few ice cubes in the reservoir once or twice a day to cool the water down a few degrees.  This is especially good for your plants if the AeroGarden's ambient room temperature is in the low 70s (that's warm for lettuce!).