I just recently flew through that part of the world, and I must say with the crazy amount of changing airspace it is not hard for a jet and ATC to have some pretty big communication issues. weak radios, most controllers speak very minimal english and only enough to get by. and they usually speak their native language when communicating with pilots that also speak it. so it is hard to tell what other jets are doing and their intentions.
I heard a jet and ATC get into it because he flew into Japanese air space yet the pilot spoke very poor english along with the controller and did not speak Japanese.
So that is my response to the how can ATC loose a jet, it is a lot harder than many think.
now for a jet to completely go black, the pilot would have to turn off all radios, radar, and stop squaking. squak basically sends out a signal that gets detected by other jets and ATC.
that is highly unlikely to happen without someone noticing.
The sad part is is that if a jet were to completely just fall off the radar immediately, it more than likely had a manifold fail or something else that caused it to explode in the air.
Even if the pilots were to both go hypoxic they would still be on radar and squaking so ATC could follow them until the plane landed/crashed.