Periodic loss
Evergreen leaves function for several years but not indefinitely.
All evergreens shed leaves periodically as stems thicken and
age, the shedding of some older leaves often occurring with
the resumption of growth after a resting period.
Sporadic loss
Discoloration then loss of the oldest stem or branch leaves during the growing season indicates the probability of insufficient feeding or a root problem. Check for signs of brown, decaying roots (waterlogging; overwatering) or matted root development (needs moving to a larger pot).
Gradual loss
Leaf tips turning brown and progressively dying back indicates insufficiently thorough or regular watering (underwatering). High temperatures and poor ventilation may be contributory factors.
Sudden loss
Ficus benjamina is very light sensitive. If moved to a much lighter or darker position there may be rapid and considerable leaf drop. Plants will usually adapt to their new surroundings, producing new leaves after a few weeks. Moving from one room to another or bringing from a nursery glasshouse to a poorly lit room may be sufficient to trigger this kind of leaf loss.