I remember at the nursery where I got this plant, I was advised to be careful with the roots. It hates being disturbed too much. So I was careful during repot, not teasing the roots during repot. I often spread the roots of most of my new plants to help them..but with my bougainvillea I did not do that, heeding the advice of the nursery folks.
Saw this article about the bougainvillea roots:
http://www.ehow.com/about_6706...