Anybody can say me why I can't use volumetric light when there is something behind?... Example : I put a volumetric light alone in my scene, no problem, when I render the scene I can see the volumetric light, but  if I put the same light into a room for example, the light works, but NO volumetric light. It seems vomumetric light needs sun light to work, as if volumetric light was dust or smoke (see picture attached) !!! I try lot of seetings but nothing works

Thx for your help.