Greetings!
I have encountered a similar problem. I obtained some water hyacinth -- which I believe may also be referred to as "water lettuce" sometimes -- and I was warned that the stuff could become invasive. There was an initial minor degree of bloomimg (small, light-lavender-colored flowers), but the plants slowly turned brown more and more, finally dieing off completely. It was suggested to me at the time that perhaps my water did not contain enough nutrients to support the growth of those plants, but I just do not know. Because the pond was primarily designed for fish, I could not simply dump liquid fertilizer into the water.
I would very much like to re-introduce a floating plant such as the hyacinth into my up-coming pond build, but I sure wish I knew what those previous plants were getting too much or too little of which resulted in their death. I live in San Diego County, so I doubt that sun and temperatures were the issues -- I am thinking it was the water itself.