Answer: The most common reason for a hibiscus to lose its leaves is the soil being too dry, or, the plant is getting TOO much water, or is in a drafty area. Hibiscus are very picky about watering, it must be just right. They also do not appreciate any drafts at all, hot or cold. They should be kept out of doorways, away from drafty windows, and air vents. They also like to have their leaves misted occasionally.
If you're giving both plants the same care and they're enjoying a draft-free location, you may want to unpot the unhappy hibiscus and repot in fresh soil. Repotting alone might provide just the right treatment to help get your plant back on the road to health.
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