|Leaf loss on privets on left side of fence but not on right side.|
|Leaf loss on privets on left side closerup|
|No leaf loss on Texas privet on right side of fence.|
The second problem is the type of mulch I think you are using. Rock mulch returns nothing to the soil. As plants grow and are pruned and the prunings taken away this takes alot from the soil. Simply applying a fertilizer is not adequate over time. The soil becomes depleted of the "organic matter" in the soil and many of the soil processes necessary for good plant health diminish over time to the point that the plant becomes "sick" or unhealthy. Unhealthy plants are more prone to winter damage than healthy plants. Healthy plants can withstand lower winter tempertures than "sick" plants. The first sign of low temperature damage of evergreens is the damage to leaves and stems and leaf and sometimes stem death and the leaves fall off.
To correct these problems you need to probably remove the rock mulch and put down first, compost and then follow it with coarse wood mulch that will not blow into your spa/pool. Bark is not suitable, it will not enrich the soil enough and will blow into the water. Coarse mulches such as you get from chipping trees interlocks and does not blow easily. Furthermore it breaks down and helps enrich the soil where bark does not. It is only decorative.
Unfortunately I think the damaged privets may be beyond help in the near future. It will take time for them to recover. If you want to invest that type of time then someone will need to start pruning them properly and renewing the soil with amendments that have been depleted. Otherwise I would just replace the plants as they go downhill and maintain a schedule of plant replacement over the years as the plants start to look ugly. Hope this helps.