Sign in with Apple Watch not working.

I'm running MacOS Sequoia 15.7.1 on an Intel iMac. I have set it up so that my Apple watch will open it rather than my having to type in my password. My watch is a Series 6 running WatchOS 26.0.2

In the past, this way of unlocking the Mac has worked, but now, although the Mac screen says "opening with Apple Watch" it doesn't do it. I've checked that we're on the same network, Bluetooth is enabled etc.

How can I crack this permanently?