My best guess is lack of water and being in a container too small for the size of the shrub. I would soak the pot with water then when the weather cools in the fall dig a large hole and transplant your hibiscus into the ground. If you must grow it in a pot chose a large container the size of a half whiskey barrel and keep soil moist. Yellow leaves are one of the signs of too little water in hibiscus but also a sign the plant needs fertilizer. Add some Osmocote to the potting soil or around the plant when you make the move. By next summer your hibiscus will be happy. Keep growing, Marianne Binetti