Answer: Light, drainage and/or watering can cause the problems you describe. It is normal for the oldest leaves to drop, but new ones will take their places if the light is right. On the other hand, sudden loss of leaves is frequently caused by a rapid temperature change. It may also be caused by such factors as prolonged hot or cold drafts, dry air, exposure to gas or furnace fumes or by changing the location of the plant from a sunny to a dark location. Ficus benjamina, commonly called weeping fig, frequently has sudden leaf drop when moved to a location with lower light intensity.
My first suggestion is to make sure the soil drains quickly and water doesn't puddle in the bottom of the pot. Allow the top inch of soil to dry completely before watering thoroughly. Ficus is sensitive to softened water so if yours is treated with a water softener, don't use tap water, use bottled water. Once you're satisfied it's getting the correct amount of moisture, think about the light it receives. Most weeping ficus require bright light and even appreciates some direct sunshine. A final suggestion is to prune it back a bit. Pruning will encourage new growth and help your ficus become bushy and compact.
Hope this information is helpful!
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