I dont really know the source of your problem, but I can understand the frustration when the preview looks nicer than the final render. I had similar problems when I was experimenting with caustics. In the preview I had some spectacular rainbows and when rendered is was...well not spectacular.
It seems that in the preview it sometimes concentrates the lighting effects whereas in a bigger render it gets spread over many pixels and thus isnt as effective.
What was supposed to cause the effect? Perhaps it higher settings?
Sorry I cant offer a practical solution, maybe a guru will come along:)
And welcome to v7complete!

"I paint that which comes from the imagination or from dreams, or from an unconscious drive. I photograph the things that I do not wish to paint, the things which already have an existence."
Man Ray, modernist painter
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