Advice is difficult without a make or model description - and even with that information advice is by no means certain.
My guess is that you have a Dimplex Optimyst fire only because you spell mist as Myst. This Youtube video may help...
https://www.youtube.com/watch?v=oJcJa003SM4
He seems to be suggesting that the mist comes from water and passes through a ceramic disc and that the discs get blocked with calcium and become less efficient.
Find your discs if you have them and clean them with vinegar or better still with a solution of citric acid - harmless and readily available as a powder.
But all this is just guesswork...
Good Luck...
March 2017
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