This is one of the most frequently asked questions about bougainvilleas and often the answer is that the plant is not getting enough sunlight. Bougainvilleas need at least 8-10 hours of direct sunlight every day. Keep them on the dry side – too much water will promote root rot and cause leaves to drop. Be careful not to overfertilize, since overfertilizing will add growth, and may inhibit blooming. Also, remember that some varieties of bougainvillea bloom mostly in the spring and fall in response to short days and may bloom sparsely during the summer.

 Additional Resource

http://okeechobee.ifas.ufl.edu/News%20columns/Bougainvillea.htm