Looks like you start out 1 cup of must with 1 cup warm water, pitch yeast, and increase the amount of must and pitch that. Not sure how to tell what the acid is. Yeast nutrient/energizer may help as well.
from:
http://winemaking.jackkeller.net/problems.asp
Stuck Fermentation: A stuck fermentation is one that has started and then stopped prematurely. This is usually caused by a lack of nutrients or acid, but not sugar, or a change in temperature disagreeable to the yeast.
When a fermentation sticks, you need to begin taking measurements of the must to determine what the problem is. Often these will reveal an obvious problem--very low acid, for example--but on rare occasions there may be several things wrong and none of them obvious. Always correct an acid deficient must with acid blend as opposed to citric or tartaric acid alone. It doesn't hurt to add yeast nutrient (1/2 teaspoon per gallon of must) and yeast energizer, too (1/4 teaspoon per gallon of must is sufficient).
After correcting perceived deficiencies and bringing the must to 70° F., wait three days for the fermentation to restart. If it does not, set aside 1/2 cup of must and add to it a cup of warm (100¦ F.) water. Over this sprinkle a good yeast known to do well at restarting stuck fermentations, such as Red Star Premier Curvee (also known as Prise de Mousse) or Lalvin K1-V1116 (also known as Montpellier). Cover the sample and allow up to two days to begin fermenting (it will probably start fermenting within hours, but give it time if it doesn't). When the fermentation is vigorous in the sample, add 1/2 teaspoon of yeast nutrient and another 1/2 cup of must from the bulk batch. Stir the sample to dissolve the nutrient, recover, and set aside about 6 hours. If fermentation is still vigorous, add another 1/2 cup of must, recover, and wait 6 hours. If fermentation is still vigorous, gently add half the starter to the bulk must so the starter sort of lays on top of the must. Do not stir. Wait 24 hours and stir shallowly. Wait another 24 hours and stir deeply. If must does not ferment with starter added, add another 1/2 cup to the remaining starter and recover. After 6 hours, add another 1/2 cup of must. Wait 6 hours and follow directions for adding to bulk must.