Check the stems for signs of scale. Otherwise, this sometimes happens with drought, or sudden cold, and when leaves are getting ready to drop. Certainly it looks as if it could benefit from the application of the some ericaceous feed - Miracle Gro make something called Azalea and Camellia food (think that's what its called), so apply that at regular intervals, according to the instructions on the pack. If the soil is very dry before application, water well first.