This just started recently. I have tried to turn Sir on / off on both iPhone and Watch, start on both devices, un-pairing Apple Watch and then set up as new device, delete and factory set Watch. No improvement.
Siri works fine on my phone, but doesn't work at all on my clock. I can activate Siri by tapping on the crown or by pressing the complication, Siri will start, say it listens, so just say "Keep on" but never do anything. Sir does not start by saying "Hi Siri" or by raising my wrist even if both options are selected.