There are two common problems that could cause this.
One) the bulb is out of range of the nearest zigbee repeater, so the messages just aren’t getting through. This is a common problem with bulbs inside closets and pantries, where the signal got through well enough when the door was wide open and you first paired the bulb, maybe because you had to make room for latter, but in later use the door only gets open partway and it ends up blocking the signal somewhatand you lose messages.
So the first thing to do is to figure out what is the nearest zigbee repeater, and see if you think the bulb should be within range. If so, you may need to do a zigbee heal to get the bulb to use that repeater. It’s easy. Leave all your other devices on power, and then unplug the hub and take any batteries out of it. Leave the hub off power for at least 15 minutes. That’s long enough for the other devices to panic because they can’t find hub. Then once you restore power to the hub, all the other devices will rebuild their neighbor tables. This could take a while, so you may not see improvement until the next day, but you should end up with more efficient routing based on your current device placements.
- you’re running into the “device unavailable” problem which is affecting a lot of people, but I don’t know if we know the reason yet. Definitely report it to support.