This is a good example as to why you should always do a yeast starter. If you don't do one and fermentation does not start right away, you don't know if the problem is temperature, the must chemicals, or just dead (non-viable) yeasts.
If you prepare a yeast starter, you will know within 15 minutes if the yeast are viable, so you can eliminate them as an issue.
Heat about 1/4 cup of water to 104F, never higher. Sprinkle the yeast on top of the water and let them set for about 15 minutes. They will start to sink within the first few minutes. After about 15 minutes, very lightly stir the solution. Let the temperature of the solution start falling on its own toward room temperature.
Soon after stirring, if not before, looking straight down into the solution, you will start seeing what looks like small explosions, then a little later you will start seeing some light foaming. These indicate the yeast is alive and well.
After the solution is about 20-30 minutes old, add a tablespoon of your must to the solution; rotate the tablespoon once to lightly distribute the must. Within a very short time, the yeast will start processing the sugar, so more foam will be produced. If you don't see this foaming, the yeast are most likely not viable and need replaced.
(The rest of this is not rocket science; the idea is to slowly add must to the solution to acclimate the yeast to the must and get the temperature of the solution down to be close to that of the fermenter of must.)
Wait 15 minutes and add another tablespoon of must. After 15 minutes, add 2 tablespoons of must and do so every 15 minutes until the temperature of the starter is within a few degrees F. of your fermentation container of must. At that time, pour the starter into the fermenter on top of the must. Give it a very light stir.
