Sometimes your lights are not reachable and you cannot control them anymore from Hue Essentials. This does not look that smart and it is the last thing you want!
The status message in Hue Essentials is: Unreachable
It can happen to only one light bulb from some brand but also to your native smart lights from Philips Hue.
There are a couple of reasons and solutions when you encounter this behaviour:
Your light was switched off manually through a classic, non-smart, switch.
Resolution is to switch it on again and be patient. If needed, pair the light again in Hue Essentials.
Someone powered off the light from the mains.
Resolution is to keep it powered (if possible) and follow previous steps.
The Zigbee mesh network cannot reach your light because the distance is too long from the bridge through the smart lights to that particular light bulb.
Resolution is to add one or more lights in between.
Some other smart lights were powered off in the house; the mesh network has trouble finding the light bulb, which you could not explain, because its path it broken.
Resolution is as simple as keep the lights powered and only switch them off through Hue Essentials or through usage of smart controls.
Interference in your Zigbee network from your own or neighbour’s WiFi (2.4Ghz). WiFi operates in the same range as Zigbee.
Resolution could be to change the Zigbee channel. Alternative is to place the hue bridge in a complete different location in your house; this location should ideally have less interference from being next to your WiFi router but should have a direct network and power cable. Last but not least you could try and eliminate usage of the 2.4Ghz WiFi network and see if all your devices can use the 5Ghz instead.
Your smart light is connected to a traditional dimmer. This results in less power to the smart bulb.
Resolution is to remove the dimmer completely. Putting it on the highest available power will not do the trick. Usage of a traditional dimmer can even cause flickering of light and sudden power-on behaviour of your connected smart bulbs.
There are certain brands that do not indicate the status whether the light is reachable or not properly. We have seen this behaviour with certain power plugs for instance.
Resolution: There is not much you can do against this but to buy better trusted (and often more expensive) brands.