Answer: There are several possible reasons for why there are problems. These plants like a soil that is not overly moist, so it is possible that they are being overwatered. Your goal in watering is to keep the soil evenly moist but never sopping wet. Sometimes, frequent but light waterings fail to penetrate the mulch layer or simply do not provide the deep soaking needed to encourage deep roots. The only way to tell if and when you need to water is to dig into the soil with your finger and see. After you water, wait a few hours and then dig down to see how effective your watering actually was -- or wasn't. Sometimes it is surprising. Keep in mind also that rainfall, cool or hot weather, wind or humidity can all affect the need for watering.
Sometimes plants fail to root out into the native soil. This can be due to over or under watering, or to incompatibility with the soil, or to being rootbound where the roots are growing in a circle and will not branch outward. Proper planting technique would counter this last possibility by spreading out the roots. If this is not done, there can be long term consequences.
Another possibility is that there is a pest problem such as scale insects. These would be visible in certain stages as a fluffy white substance along the stems during the summer, and would cause leaf drop and die back. Scale can eb treated with horticultural oil or insecticidal soap applied and reapplied according to the lable instructions, but it is somewhat difficult to control if the problem has become severe already.
Yet another possibility is overfertilizing, since this can burn plants and cause leaf drop. Unfortunately, it is very difficult to diagnose these types of problems long distance. I would suggest that you consult with your county extension and possibly with professionally trained nursery staff where you purchased the plants to try to determine what is causing the problem and how to proceed from here. I am sorry about your plants not doing well.
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