First make sure the plant is getting enough water. Droopy leaves are a sign of thirst but also of too much water. If your pot has poor drainage or sits in drainage water it could be drooping due to root rot and too much moisture. Poke your finger into the soil up to the first knuckle. If the soil is wet you are overwatering. Also, mandevilla is a tropical plant. A cold night below 50 degrees can damage foliage and cause the plant to droop. Good luck and Keep Growing, Marianne Binetti