Kinda Strange. I open a default new scene, thicken the fog, set the Source light to have shafts on some pillars I add, it works. Turn that off add a Directional Light. Same settings. No shafts. What am I missing?
-Which version do you use?
-do you use an atmospheric or exponential fog?
-could you post your settings so that I can test it 
4.7.1
Here is Exactly what I do:
File/New/Default Blank Scene
(everything here is included in this scene but the pillars)
Add a cylinder, Sale it tall and thin, clone it a few times to make a row.
Rotate view so the sun is in the BG.
Increase Density Multiplier of Atmospheric Fog to 10.
Turn on Occ, Shafts in the “Light Source” properties. This works, so does the Bloom Option.
Turn both Occ and Bloom off. Turn Source LightDensity to 0.
Add a directional Light. Rotate it so that is faces the same way and casts the same shadows. Invoke shafts and nothing. Incidently if I turn the bloom option on the directional light at this point. A shaft of light comes down from the sun disc.
Exponential fog kinda works but not really, could be the settings. In Both cases the Bloom comes directly out of the sun disc and does not interact at all with any geometry.
The only thing that looks right is the “Light Source”.
Clearly I am doing something wrong.
Sorry to bring this thread back from the dead but did you ever solve the problem? I’m having the exact same issue in 4.14.1 and can’t seem to figure out why. Everything was working great until yesterday morning when I opened my project up and neither of the light shaft types would work. I’ve been trying everything to no avail. I’m think of rebuilding the engine but I can’t imagine that would be the problem.
Same problem here, deleted the base source light, using a directional light with same settings, no Light Shafts appear with this one…
Check the Engine Scalability settings and set the Post Processing to High.
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Guys ,
If you are using HDRI , get rid of it.It will be too difficult to see in bright light background.
this worked!