The cause could be cultural (watering, fertilizing, etc.) or improper site conditions. I would review the following article to ensure your mandevilla's needs are being met:
Temperature could also be to blame - specifically temperatures that are too cool. Has your plant been exposed to cold temperatures (below 50F)?
Are there any signs of an insect infestation, such as aphids? Information on aphids can be found here: https://www.gardeningknowhow.com/plant-problems/pests/insects/homemade-aphid-control.htm
For information on how to get a mandevilla to bloom, please visit the following link: