If it is not the water, temperature, exposure to chemicals, dryness or sun, it could be the nutrients available to the plant. A lucky bamboo will eventually need nutrients to do well. Do you give it a very light feeding occassionally? You can also check for any signs of disease. Is the stem also yellowed? Are there any signs of rotting or mildew? Do the roots look healthy?

Here is a link that might be useful: Yellowing Lucky Bamboo Leaves

Try to add micronutrients, your lucky bamboo plant may need these when you have grow them for such a long time, the nutrients inside their body had been almost gone out.

I've had the same problem, and someone at work told me to feed with a weak solution of alfalfa "tea". I let some alfalfa cubes (used to feed horses, etc) soak in hot water and then added more water, and poured some in the container holding the lucky bamboo. It has helped to the extent that the plant has new growth and isn't yellowing anymore. This is not too exact or scientific, but it did seem to help.