If fermentation is languishing, first ensure that the temperature is at a suitable level, between 18 and 22°C for whites and between 24 and 30°C for reds.
Then, if the nitrogen inputs have been insufficient, nitrogen needs to be added, usually in the form of amino acids via yeast autolysates. Lastly, the must can be oxygenated.
In the event of stuck fermentation, the cause must be identified. Most often, the cause is elevated temperatures, a lack of nutrients or oxygen, or an excessive alcohol concentration (some strains of yeast do not tolerate high levels of alcohol).
Stuck fermentations are dangerous. In many cases, it is necessary to inoculate using yeasts that can withstand high alcohol levels and low quantities of sugar. The protocols for the use of these yeasts are provided by the manufacturers.