Is the black one definitely a Sonos One, and not a Sonos One SL? (i.e. does it definitely have a microphone on top)?

Yes definitely Sonos One Gen 2 with microphone and responds to all other Alexa commands but doesn’t receive announcements from Alexa routines 

One suggestion to try (may or may not solve it, but does no harm to try):

• Turn off the router and your Sonos Ones at the wall.
• Wait five mins.
• Turn on the router and let it boot up fully. 
• Turn on the first of your Sonos Ones and let it boot up to white light. Then turn on the next, and the next, letting them boot up each time.

Then see if the rogue speaker starts receiving announcements...  

Are they all Gen 2 speakers? (I don’t know if that makes a difference though.)

You might also remove Alexa from the speaker and re-add it.

I tried that but no difference.

Did you try what I suggested…?

Not had a chance yet as several of my systems need a controlled shutdown first.

Just curious…..

By announcements you mean for example:

• You: Alexa announce
• Alexa: What’s the announcement?
• You: Dinner is ready...come and get it!
• Alexa: Announcing...
• Alexa: (In your voice) Dinner is ready...come and get it!

Tried the router turn off but no change.

I am using announce in a conditional routine and not interacting with Alexa by voice command

Found a work around by using ‘say’ instead of ‘announce‘ within the routine.

So you have a routine set in the Alexa app to make an announcement on a schedule of sorts. I’ve found Alexa to accept certain words and suddenly one day she does not making it necessary to change the word. I think it’s something in the algorithm. But what do I know? Glad you found the solution.