I agree with gratus. Your likely problem is an acidic must - not an ideal environment for your yeast. Per Jack Keller:
Calcium carbonate reacts preferentially with tartaric rather than malic acid, so one should not try to reduce acidity more thab 0.3 to 0.4% through its use. A dose of 2.5 grams per gallon of wine lowers TA about 0.1%. After its use, the wine should be bulk aged at least 6 months to allow calcium malate, a byproduct of calcium carbonate use, to precipitate from the wine. The wine should then be cold stabilized to ensure tartrate crystals do not precipitate out after bottling.
Potassium bicarbonate is used to deacidify a wine with a low pH (below 3.5), but should not be used to reduce acidity more than 0.3%. A dose of 3.4 grams per gallon of wine lowers acidity by about 0.1%. After use, the wine should be cold stabilized, as up to 30% of the potential acid reduction occurs during cold stabilization. It will cause a greater rise in pH than calcium carbonate for an equivalent reduction in acidity.
Finally, potassium bitartrate (a.k.a. Cream of Tartar) is used as a catalyst to help promote cold stabilization. It promotes the formation of tartrate crystals and is used at the rate of 2 to 5 grams per gallon, followed by vigorous stirring. Its use results in better and quicker stabilization, and these benefits will occur at slightly higher temperatures than without it.