If the stem is still hard and not black and squishy, it may just be going dormant. If so it should start growing new small leaves at the tip of each branch. These plants do drop their leaves each year, go through a dormant period and then start growing for the new season. If your plant had come out of dormancy and dropped new leaves, I would suspect the plant is over watered. You do not water or barely water (cup a month)when a plant is dormant. Dry it a bit and begin watering when you see new leaves but "little leaves, little water, more leaves, more water." They are like succulents= don't like to sit in wet soil. If this is not it, your soil may be too dense. Bill