I recently bought some Mandevillas and they are growing quickly; however, the blooms are not opening. What can I do?
Check your growing conditions first -https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm
You might try adding a little phosphorous in the form of bone meal. Otherwise, just sit tight. It often takes newly planted bushes awhile to establish enough roots to be able to concentrate on flowering.