Mandevilla Flowers Fading. Flowers may also change colors or fade. Its common for these diseases to cause the foliage to drop off of the plant reduce flowering and turn the foliage yellow.
It could be that it is just a little underwatered just enough so that it cant support its flowers and so it drops them. Other causes of faded flower color include the fact that flowers generally fade after pollination. These are tiny pests that attack the base of buds and cause them to fall off.
The bright clear pink flowers stay true to color never fading in the hot sun like some Mandevilla.
In order to get the best mandevilla flowers throughout the summer give your mandevilla plant a high phosphorus water soluble fertilizer once every two weeks. Some varieties die back to ground level at the first frost. These are all bushy forms of Mandevilla and are ideal for pots containers and hanging. Treat as an annual in cooler regions.