One of the most common mistakes of the amateur alchemist is taking shortcuts.  Many alchemical formulas appear overlong and unnecessarily convoluted, but only an experienced practictioner can determine when - and how - to abbreviate the process, if it can be done at all.  Alchemy is not just about the combination of physical materials in a specific manner, but about how they are influenced by invisible forces, such as the movements of stars and planets, and these things cannot be rushed or altered.  If a formula states it requires a year and a day of low heat and counterclockwise stirring to prepare, you had best allow a year and a day to tend your cookfire.