Often times the easiest way to find the answer to a question is by inspecting the OP's blendfile.
I personally try and ask for a .blend in a way which describes a few possible causes (in case it turns out one of them is the cause, then the need for a .blend is avoided), or just saying something like:
There are many possible reasons this could be happening. The easiest way for us to determine the cause is by inspecting your file.
Is it possible you could upload your .blend?
Acknowledging that some users might not be able to share their file.
Is this a good idea? Or does it create to much noise?