Answer: Based on your description I am not certain what is happening to your plants, but I suspect there may be a problem with watering.
The tropical hibiscus is a sun loving tropical plant. It needs as much full direct sun as possible, preferably all day long. In less sun, it will go through an adaptation period while it adjusts to the lower light. This can cause some leaf loss.
You can keep this as a houseplant during the winter. Bring it inside in the fall before temperatures drop belpow about 50 degrees. Give it as much sun as possible and keep it out of drafts--hot or cold.
Both of these should be watered so the soil stays evenly moist, meaning damp like a wrung out sponge. Use your finger to check the soil and see if you need to water. When you water, make sure it is soaking into the soil and not running out between the soil and the side of the container. Also, after watering, empty the drainage saucer if there is one.
Both of these can also be fertilized regularly using a water soluble fertilizer such as 10-10-10 plus minors. Or, you can use a slow release granular fertilizer with the same numbers. Read and follow the label directions for how much to use.
I hope this helps you trouble shoot.
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