It is only natural for your perennial to shed some buds here and there but if you notice a lot of loss it could be a couple of reasons. Mandeville likes full sun, however, too much sun, especially during the hot summer months, could result in loss of buds. If you notice the foliage and flowers looking a bit bleached, shift your plant to an area that receives full sun only in the morning and not during the bright mid-day sunlight.
If you decide to bring your Mandevilla indoors during the cooler months, it will inevitably lose buds due to the shock of the transition. It is best to prune your plant when doing so to prep it for dormancy. Also, too much water can cause your plant to lose buds. Remember to only pot your plant in a container with drainage holes. If it is planted, never allow the roots to sit in standing water.