Answer: It sounds as though you're doing everything right in terms of care for your ficus. Two things to look for - first, the leaf drop might be normal. Although evergreen, ficus will drop its older leaves in preparation of developing healthy new replacement leaves. If it is still getting new growth at the ends of the branches, the loss of older leaves is normal and nothing to be concerned about. The second thing to look for is a population of insects. Spidermites can cause yellowing, dropping leaves due to their feeding. Look for webbing along the leaf stems where they attach to the branches. If there are no spider mites, given the good care your ficus is receiving, I'd chalk up the leaf drop to natural behavior.
Best wishes with your ficus!
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