Why they hear ping and why now they don’t?
Ping might have been generated from a Iranian Kilo spoofing a ping to keep CNN continuously cover speculation about the missing airplane.
Assies said ELT only work about 50% of the time according to their study? I dunno about that. Who are they? Do they manufacture ELT? Nope.
ELTs work only about 50% of the time in crashes on land, and an ELT is the first notice of a crash on land only about 15% of the time.
When everything goes right, they are effective for land crashes.
https://www.atsb.gov.au/publications/2012/ar-2012-128.aspx
The fixed airframe ELT on an airliner is not designed or meant to work in a water landing.
The portable ELTs and the slide/raft ELTs would be expected to be used by survivors.
There are a couple of portable ELTs, and an ELT with each door slide/raft, on a 777, in addition to the fixed airframe ELT.
Had the plane landed successfully somewhere, or successfully ditched, without foul play, we should have heard an ELT.