Here is a question that was posed to AG and their answer:
Why are my salad greens pale and spindly?
By Chris Hayes
Lettuce is known as a "cool season crop" because it thrives in cooler temperatures. If it's too warm, it won't grow properly; it will be spindly and floppy and it may "bolt", meaning that it sends up a long stalk in order to go to seed. Lettuce will do this under one of two circumstances: either when it experiences repeated daytime temperatures above 72 degrees, or when it is nearing the end of its natural lifespan. If you are growing lettuce in your AeroGarden, place the Garden in an area of your home that does not regularly get warmer than 72 degrees.
Unfortunately, once lettuce has bolted, it will never be short, crisp and bright green again. Some people like to grow Salad Greens in the cooler months, and then a "warm season crop" like Chili Peppers or Cherry Tomatoes in the warmer months. AeroGrow’s Romaine Seed Kit and Heirloom Lettuce kits can withstand higher temperatures (up to about 76 degrees) without bolting.
Another cause of spindly, pale lettuce (or any plant) is lack of light. First, be sure that the grow light hood is just a inch or two above the plants for optimal light. And if your compact fluorescent grow lights have been in use more than six months, they are losing their growing power even though they may appear to be quite bright. We cannot detect the subtle loss of full spectrum light, but your plants can!