I sent you a photo of the bougainvillea leaf but may have sent it to the wrong section. I have had my plant about 10 years and never know what type of compost to repot it in. I gave it some rhododendron feed and the result is yellow leaves with dark green veins. Also very few leaves. It is flowering profusely and is a wonderful sight at the moment. Would B@Q peat free compost be good or multi purpose with peat? Is it acid loving or alkaline? We grew them with either in Kenya I think.
Use a general liquid plant feed … the leaves exhibit magnesium deficiency, which can be caused by too much potash, which might account for the leaf fall and the big show of flowers.
When re-potting use a mixture of half:half garden soil and compost. The soil helps to even out minor nutrient deficiencies.