It is not unusual for an older Christmas cactus to drop leaf stem (their branches or "clades") or appear somewhat limp at various parts of its growth cycle but newer, younger plants should be very green and perky. If yours has limp leaves for an extended period of time or you find the buds fall off before the plant blooms, over-watering is the most likely cause. The plant may also be unhealthy for other reasons. Look for any signs of insects, disease, or distress. The plant should be robust, deep green in color, soil should be moist (press your finger tip in to check) and free of mold. If you find anything suspicious, isolate your plant from other houseplants and investigate the problem. Some things are harmless, others can spread to your other plants.