What do yellow leaves mean on Rose of Sharon plants that you're watering well? Perhaps you're watering your rose of Sharon plants too well!
Over-Watering is as Bad as Under-Watering
Over-watering plants is an extremely common problem, and one of the signs is yellow leaves (when they should be green). The roots of plants not tolerant of waterlogged soil won't be able to "breathe," and they die of a lack of oxygen. Ironically, after the roots "drown" in this manner, you'll see the "drying up" of the leaves because the now debilitated roots won't be able to make use of all that water.
Drainage is Key
You may truly think that you're not over-watering; but if your soil isn't well-drained, the roots still could, in fact, be resting in waterlogged soil. If your case is, indeed, a case of poorly-drained soil, your best recourse would be to transplant the rose of Sharon plants to an area where the soil is well-drained. Prepare the area now, in the summer (peat moss can be mixed into the soil to improve drainage), and do the transplanting in autumn, once the weather cools off.
Why Only Some Plants are Dying
Are some of your plants thriving while others are showing yellow leaves and dying out? What seems a 'random' dying out might not really be so, at all. Let me draw an analogy. In a group of 20 people who smoke tobacco, some might die of cancer in their fifties, others might do so in the seventies, and still others might live to a ripe old age without becoming cancerous. We aren't all alike, and even folks with very similar genetic makeups are subject to different environmental influences. To some degree, the same is true of plants. For all you know, the rose of Sharon plants that have survived till now may have gotten off to better starts as 'babies' and are simply a bit more resilient than those that have already died from the over-watering.