The symptom you described is usually cause by one of two things.  The first is overwatering. The soil needs to be lightly moist but not soggy wet all the time. If the soil is always very wet and filled with water, the roots can’t breathe and drown just like we would underwater. Plants need some water to replace what they lose to evaporation but too much and they drown. Also, if you piled dirt up higher on the stem when you re-potted it that will cause the same symptom as drowning (leave turning yellow and falling off). Dirt piled against the bark cuts off the flow of moisture and nutrients from the roots to the branches and leave above. Brush away any soil from the stem until you find the original soil level and let the trunk breathe. Most folks think they are doig the palnt a favor by adding more soil around the stem but the opposite is actually true…it slowly strangles the plant. Hibiscus like to be fed in the summer but not at all once they come inside from Oct.-March. They will also use only a tiny fraction of water once they are brought inside as well.