The ELT attached to the fuselage was likely destroyed on impact.
They only work about 50% of the time anyway according to an Australian study I read..
The ELT also will not work at all if submerged.
An ELT must get it’s antenna out of the water to be heard.
So, despite the fact that there were probably 10 ELTs total on the 777, it’s not unusual at all that none of them were heard.
Modern ELTs use 406Mhz as the primary beacon, and only have a low powered 121.5mhz beacon for local homing.
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.