I just purchased a white and red mandevilla. It has blooms but don't open they fall off. I have a red and pink open and they don't have this problem. What could be wrong.
Too much or too little water can cause mandevilla buds to fall off. The soil should be kept consistently moist, but not waterlogged (if the soil gets too soggy there may be a drainage problem with the soil mix). Pest problems can also cause this- check inside and around the flowers for tiny insects such as thrips or aphids.