There are several reasons that a plumbago plant might not bloom, but the most common are inadequate fertilization or that the plant is not getting enough sun. If the plumbago plant is healthy, the problem is probably that it is not receiving direct sunlight for enough hours in the day.
The plumbago is a tropical plant, so it does best in the sun. It can thrive in partial shade, but is not as likely to flower. Plumbago plants do well in pots, because the plant can be moved in and out of the sun and can be brought indoors during colder weather.