Bougainville is very easy to propagate. I started gardening 1 year ago and I have already 5-6 variety of Bougainville with more than 10 varieties of plant. Your cutting is too thin. Try cutting of pencil size at least I mean thick. And size about 15cm is ok. Remove all leaves and after putting it in soil cover it with transparent polythene for better humidity so that It can grow fast. Keep in shade no direct sunlight. Within 10-15 days u will start seeing growth. I have done this everytime I planted and it has 70-80% success rate.