Whenever planes are lost over water, which this seems to have been, we have to remember that the oceans are HUGE and lighting is, shall we say, poor or non-existent for at least 10-12 hours of the night! Debris submerges into dark and deep waters, and water currents move things around. Various countries and air agencies have to cooperate, combine technologies, and deal with realities such as a black box that might not have survived any explosion (including impact with the ocean). I wouldn't think it surprising at all that someone didn't see it happen given the remote areas of Asian land and water masses.
I remember watching the space capsules come down in the Pacific, and it still took aircraft carriers, helicopters, radar, and many other efforts to find them - and that was when they knew exactly when and where those capsules were expected! When we see Coast Guard cutters and planes searching for a missing ship, we get a glimpse of what it's like to create a grid over open water and then patrol in a criss-cross fashion. It's easy to miss something even when you have an idea where it was last seen or had some communication from the people on board. In this case, we don't seem to have that. I'm not at all surprised that they can't find it yet. I hope they will soon so we have some answers and the families get some closure.