Mandevilla that do not bloom may not be receiving the proper care or may not have adequate growing conditions. Inadequate light, insufficient water or food and poor soil drainage can prevent a mandevilla from blooming.
According to Home Guides, the number of blooms a mandevilla produces depends upon the amount of sunlight it receives. For the greatest number of blooms, full sun is required. Moving a mandevilla to a spot where it receives 6 to 8 hours of direct sunlight a day can increase blooming.
Without the proper water or nutrition, a mandevilla's energy stores are lowered. This can reduce flowering. If located in a hot, sunny spot, mandevillas require sufficient water. Do not allow the plant to dry out. Check the soil daily, and make sure it is consistently moist. During the growing season, mandevillas require more fertilizer. Apply a water-soluble fertilizer that is high in phosphorus. Follow the manufacturer's instructions for specific application rates.
Well-drained soil is essential for good flowering. High-quality soil includes equal parts of potting soil, peat moss and builder's sand. Place the plant in a container with plenty of room for the roots to spread and adequate drainage holes.
Mandevillas do not bloom in the winter. Do not fertilize the plant during this time. Water only when the soil is dry to the touch. Resume regular fertilization and watering in the spring.