Echo Dot stopped receiving calls

[johnf52]

Hi
As a family unit we have three Echo dots in each of three accounts.  All nine units are actually on the same network as they are within the same building.
Everything has been working fine for over a year until suddenly one or two of the "Dots" have stopped receiving Alexa to Alexa calls within these groups.  eg if Karen, upstairs, calls me from one of her Dots, two of mine downstairs will respond but the third, cheerfully continues playing the radio station, or Spotify, or whatever, whereas previously, it would interrupt itself to say "Karen is calling".  I reiterate that Karens dots and my Dots have separate Amazon accounts, but are connected to the same network.  There are identical issues with the third member of our family Barrie.  The dot that does not work is always the same one.  This issue does NOT randomly move from Dot to Dot.
The Dot that does not receive calls is quite happy to make them and respond to all other commands (Time, weather, calendar, read my book etc etc )

Does anyone have any idea what is going on and how we can get our "Alexa Network" back on task?

Thanks