Problem: I have 2 x sonos play 3 and neither of them will factory reset.  Having changed my router, I had to reset both, but they never went out of flashing amber light (no green!). 

Solution. Yes!, I finally connected the speakers, one by one, to the router using an ethernet cable. I made the reset procedure and wait...wait for at least 7 minutes, until green. Then, disconnected the ethernet cable and, after that, added to the system. In one case, an update was applied to the speaker.

Great!: I have now both working again.