There are two general reasons a must doesn't start fermenting. First, sugar level to high for yeast to start, it takes a hydrometer reading to know. Second reason is due to the Ph Level of the must being outside the ranges that yeast are happy in, usually to low. It takes a ph meter or at least ph strips to know. Yeast are happiest if the ph is between 3.1 and 3.7-4.
Also, some musts take longer for the yeast to fire up and take off than others. 48 hours is suggested to wait, at least, if you just sprinkled the yeast on top. I always start my yeast waking up in warmish water, with some yeast nutrient and sugar 24 hours before I plan to pitch it and it takes off very quickly.
Another side note, your mead will need to have some extra nutients added to it, even if you followed a recipe and added some at the start. Honey and flowers have little or no good food for yeast, it all must be provided.