Answer: The most common reason for a hibiscus to drop leaves is the soil being too dry, but you say that can't be the problem so we have to look elsewhere. You also don't mention any irregularities such as pests or residue/mold on leaves so we will rule those out as well. That leaves us with two other probabilities: too much water, or a drafty area.
Hibiscus are very picky about watering -- it must be just right. They also won't tolerate drafts, hot or cold. They should be kept out of doorways and away from drafty windows and air vents. Overfertilizing or buildup of fertilizer salts is another possibility. Houseplants should only be fertilized during the spring/summer months (March through August as a rule of thumb). Occasionally flood the soil with water and allow to drain thoroughly, to rinse away any built-up salts.
When bringing plants indoors after a summer outside, make the move gradually so the plant can become accustomed to the lower light levels indoors. Start by placing the plant in a sheltered spot outdoors, out of direct sun, for a few weeks. Then move it indoors to the sunniest place you have. After a few more weeks, you can move it to its winter home -- for hibiscus, that should be a place with bright light.
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