Your hibiscus looks healthy and it is blooming so those yellow leaves on the bottom could just be the result of drying out on a hot day, or suffering too much water in a down pour or even too much plant food. Fertilizing once a week seems like a lot. Too much fertilizer  stresses out the plant. A mulch on top of that bare soil and watering when the top one inch is dry will help you to offer  more consistent watering. You can pick off the yellow leaves when they appear so you can tell if new leaves are turning yellow or if it was just a single temper tantrum that the hibiscus had from a single event. Keep growing, Marianne Binetti