Have you tasted it?
If it tastes very sweet tip both demijohns into a fermenting bucket and make sure you oxagenate it. Take an SG reading
Make up a strong yeast starter.
1 heaped tsp yeast
1 tsp nutriant
2 to 3 tsps sugar
Warm water
Cover and leave yeast starter for 24 hours
If you have vit b1 tablet add 1 crushed tab to must and pitch starter.
Then wait.. give it at least 48 to 60 hours if nothing happens tip down drain.
If anybody else out there has any thoughts please add... im no expert and never had this happen to me.