This usually happens if the chocolate mixture was still too warm when you fold it into the whipped cream. Whipped cream melts and won't hold its shape when it gets too warm, and the chocolate might get grainy. It’s important that the chocolate ganache is completely cooled before adding the cold whipped cream. The chocolate mixture should also not sit in the fridge for longer than 20 minutes, or it will be difficult to combine it with the whipped cream. Another reason why your mousse might not set is not being careful when folding in the whipped cream. Don't use a hand mixer or a whisk to combine the chocolate and cream. Carefully fold them together using a spatula and go slowly.