The problem could be caused by one (or more) of several things.
> Watering. This is the usual cause of the symptoms you are encountering. A bonsai should be kept evenly moist, it should feel like a sponge after all the water has been squeezed out of it. You may not be watering it enough.
> Light: Not enough exposure to light will cause this problem.
> The roots may be rotting because the tree needs transplanting, or there is no drain hole in the bottom of the pot. Remove the tree from the pot and look at the roots. If they are black and mushy, stink and/or are soaking wet you have root rot and the tree must be transplanted to save it.
> Location: Many bonsai sold commercially as indoor bonsai are not indoor bonsai. Only tropical and some semi-tropical species can be grown indoors. All others must be outdoors unless you enjoy watching a tree slowly die. Could take up to a year.
> Could be suffering an insect attack or a fungal attack. Check the underside of the leaves for very small insects no bigger than the head of a pin or fine webbing. Check the leaves for little dots that seem soft and mushy in the middle. This would signal a fungus attack.
> If an indoor tree, be sure air conditioner or heater vents are not blowing on it.
> If an indoor tree, it may lose leaves after being moved to a new location. Ficus and serissa in particular are known for this.
> Tree is going dormant just like the big ones in the woods because of the time of year,
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