That's one of the main reasons this forum was started--not necessarily to provide answers but to actually try and figure some of them out! Truth be told, seagrasses are quite difficult: they are a much more advanced plant than macros and caluerpas. THey can do well in some systems, and completely fail in others.
The key seems to be in providing them with enough nutrients(from the sediment, since they take it in through their roots) which we can't seem to do well enough. Herein lies the problem...and the challenge...and the forum to discuss failures, sucesses, and possibilities.