There are many different factors that are involved in a healthy fermentation. Ask, “What is the specific gravity of the juice, currently?” This may help to troubleshoot the fermentation issue.
If specific gravity is still very high (roughly between 1.08 and 1.09) be sure the yeast has been properly added to the juice. If it has and no fermentation has occurred, prepare another yeast culture and add it into the juice.
If specific gravity is still very high and a healthy yeast culture has been added, or fermentation had begun and stopped (resulting in a specific gravity below 1.08 but above 1.00), check the following:
Is the juice and area at a proper temperature for fermentation (roughly 76℉)?
Is there enough oxygen present for fermentation?
Is an airlock being used during fermentation?
Oxygen is used by yeast as an energy source to ferment sugars to alcohol. If the temperature of the juice/yeast mixture and the area is in a good range for fermentation, try to agitate the mixture by stirring briefly to introduce oxygen into the liquid. If an airlock is being used it is possible that the airlock is choking off the oxygen necessary for a healthy fermentation.
If none of these common causes appear to be the issue, consult with the winemaker on other potential complications.