Sweet, yes i suppose that what kind of problems you have here it is difficult to say at once. In generally for right result as for organoleptic properties of distilled spirit are responsible both quality of grain, grain preservatives, processes of fermentation and distillation itself. Naturally, to denote and describe the reasons you want to exlude in order to get rid of negative results in organoleptic properties of the spirits, for sure, i think it is not possible. There are description of organic compounds that indeed are responsible for bitter, burning, harsh tastes, but what concretely you have to correct to improve organoleptic properties it is possible to solve the task just on place.