Could be, that your emissive part is too small to be recognized by Lumen. There is a minimum size for how big an emissive surface has to be to work with Lumen.
From the manual:
"Lumen and Emissive Materials
Emissive materials propagate light through Lumen’s Final Gather with no additional performance cost. However, there’s a limit to how small and bright emissive areas can be before they begin to cause noise artifacts. This is inherently much more difficult to solve than manually placed light sources."
You can test it by putting a normal sphere or cube into your scene and give it an emissive material, then play with it´s size to see, if/when it works with Lumen and then, if it still works, if you turn your camera away from it.
But there seems to be a new setting, that may be able to override that min size rule, something along those lines were mentioned in the Lumen thread: