Answer: Based on your description it's difficult to make a diagnosis, but it is possible that the tree is infested with mites (very small and hard to see but usually accompanied by fine webbing on the undersides of the leaves) or scale (small dark discs affixed to the leaves), both of which can be controlled with insecticidal soap applied according to the label instructions. It is also possible that the plant is objecting to the dry indoor air during the winter heating season, so you might try raising the humidity around it with a humidifier or pebble tray. This plant hates changes, so it would react by dropping leaves to a change in air temperature, being placed in a draft, a change in lighting and so on. Once it has become accustomed to the change, however, it will usually leaf out again. The last cause I can think of is under or over watering -- make sure the soil is a bit moist but not overly wet. Repotting can change the amount of water you need to apply.
Q&A Library Searching Tips