You didn't mention this so I'm not sure if you did it. But did you
rehydrate the yeast in warm water before you added it to the must? It
is necessary in order to "wake" up the yeast and let them know they
need to start doing their stuff. If you add it to the must dry then it
doesn't "shock" them enough to get active because the sugar hinders it.
They need water and oxygen to get going.