Who is to say that any ransom demand would have been made publicly? It may well be that one has been made but that we just don't know about it.
Besides, if they knew where the plane is to bomb it this would be a hostage situation right now. They might be in a country which wouldn't welcome the incursion but I can't see the Chinese being overly bothered about that given the number of their citizens on-board.
Now we all know that that isn't true so it may be they felt that keeping it quiet (for now at least) would give them a better chance of getting what they want.