I’m sorry to hear about your fermentation troubles! Indeed, it’s one of the most important aspects of winemaking to master since sluggish and incomplete fermentations can ruin a batch. Here are some things to keep in mind next time you find yourself in a battle with a balky fermentation:
Temperature too cold: All wines like to ferment above 50 °F (10 °C) so be sure your juices and musts are well above this temperature, and preferably at least 60 °F (16 °C). Don’t let red fermentations rise above 90 °F (32 °C), however.
Not enough nutrition: Some juices and musts are deficient in micronutrients like argenine or in macronutrients like nitrogen. You might want to add a little yeast nutrient (diammonium phosphate, or DAP, is better than nothing, but a mixed nutrient would be best) to your wine at a recommended dose range to make sure the yeast has something to boost them and aren’t missing out on critical nutrition.
High alcohol: Alcohol is toxic to yeast; it makes their cell walls permeable. High levels (over 15%) will almost immediately kill off many yeast strains. If your juices or grapes are very ripe, be sure to add water to the fermentation to bring the Brix down into the “sweet spot” 22–25 °Brix territory. It’s OK to add water, just try to keep an addition below 10% to avoid diluting colors, tannins, acidity, and flavors too much.
pH too low/acid too high: A pH below 3.3 can inhibit yeast growth. Check acid levels on juices and musts and try deacidification with potassium carbonate or potassium bicarbonate if needed. Shoot for a titratable acidity below 8.0 g/L.
Too much residual sulfur dioxide: Most yeast strains are sensitive to free sulfur dioxide over 15–20 ppm. While adding 25–35 ppm total SO2 at the crusher is wise to knock down indigenous yeast and bacteria (if you’re not including them/want to discourage them) try to make sure too much wasn’t added.
Use a “strong” and appropriate yeast strain: Some yeast strains are very sensitive to alcohol and sulfur dioxide. The good ol’ EC-1118 (aka Prise de Mousse) will work but there are other strains like Lalvin K1-V1116 that are especially good in tricky fermentation environments. Generally, any yeast advertised as working well for restarting stuck fermentations will be alcohol- and sulfur dioxide-tolerant, but be sure to match your yeast to your base wine’s alcohol level.
Correct yeast strain but culture not active: Be sure to proof yeast (proper rehydration) before pitching to make sure it’s active and robust. Do not use a particular batch if you don’t see a lot of activity. Try a new packet or try a new strain.
Correct yeast strain, active culture, but pitched incorrectly: When introducing new yeast into a “compromised” environment (high microbial load coming in from the field, low temperatures, low nutrition, high Brix, high alcohol), it’s important to really acclimate the new yeast well. Try feeding the active yeast small but increasingly larger bits of the juice or must, perhaps waiting around 15–20 minutes and checking for activity after each addition before pitching the expanded culture into your fermenter.
Keep in mind that a combination of the above factors could be your fermentation-limiting problem. I wish you luck diagnosing and working through your issues!