One brand of ZigBee devices become unresponsive

Again, if the angle is steep enough your couch becomes a mangled 20’ mess of wire springs and wooden frame. This goes back to wifi setups where placement is everything for good coverage. Also channel 15 is often chatty, have you checked your Zigbee diagnostics to see what your channel utilization looks like?

Is that this bit?

},
"energy_scan": {
  "11": 27.058823529411764,
  "12": 41.1764705882353,
  "13": 47.05882352941177,
  "14": 30.58823529411765,
  "15": 55.294117647058826,
  "16": 52.94117647058823,
  "17": 72.94117647058823,
  "18": 68.23529411764706,
  "19": 55.294117647058826,
  "20": 56.470588235294116,
  "21": 83.52941176470588,
  "22": 83.52941176470588,
  "23": 42.35294117647059,
  "24": 32.94117647058823,
  "25": 27.058823529411764,
  "26": 38.8235294117647
},

That’s the one and 15 looks pretty good! Better than my 15 haha.

So should I be using a channel that has a low value in that list such as 25 instead?

My current setup had defaulted to 25 when I first set it up, you are best to just check what channels are the best overall for your setup as you may need to recreate the setup to apply the channel changes when testing.

Some devices don’t like 25, more don’t like 26. You can play with them but honestly at 55 your channel 15 is pretty good - BUT look at the cookbook because there is overlap with wifi and that could still be your problem.

My understanding from the cookbook was that 15 was a good one to use.