Hi Beth,
from me too!
The white coloration on the blooms in your photo looks like water spots which can be alleviated by watering from the bottom. Yellowing leaves can be from overwatering and I'm wondering if the soil they are planted in perhaps is holding too much moisture. Some nurseries plant in very heavy soil that does not drain well, I assume to keep the plants moist while sitting on shelves in the garden centers waiting for consumers to buy them.
Mandevilla/Dipladenia need a well draining potting medium. I also wonder if they could be root-bound, possibly in need of larger containers. I find Mandevilla also does best with some shade; bright indirect light but no direct sunlight, which will burn the foliage. For more blooms, fertilizer with higher phosphorus (high middle number) will help produce more blooms but only use half strength to what is listed on the package.
Good luck with your plants and I hope they do well for you. Even though they are tropical plants you can overwinter them by moving them indoors when temperatures dip below @ 50ºF. There are several Mandevilla (Dipladenia) listed in our database with information and photos:
http://garden.org/plants/searc...
