Those symptoms point to a soil moisture problem in most cases. It is usually either too soggy wet (check the container holes to make sure they are not plugged) or too dry for a period. Both cause root death which is followed by the leaf yellowing and drop. If your plant is not too far gone fixing the soil moisture level will result in recovery and normal growth. A little application of a fertilizer for acid loving plants such as Miracid is also a good idea to assist in new growth and improved color and vigor.
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