Voice control has stopped working on my Amazon Fire TV Stick?

Voice control has stopped working on my Amazon Fire TV Stick 4K. I use it (Alexa) to control a smartplug. Voice control still works from my Alexa hub. The smartplug control still works from the Firestick when I press the on/off button in the device control page. I have changed the batteries in the remote.

I don't use it, but voice control is not working for programme search either. So the issue is that general voice control has stopped working. When I press the Alexa button on the remote, the blue light shows and the blue line appears on the TV. It just doesn't do anything.