I have several Plum Delights lining my driveway. The first two years I fertilized them (the ones that survived the fireants!) with standard fertilizer and they just didn't seem to thrive. This year I used Miracle Grow for azaleas and they look amazing! They filled out beautifully and have the deep purple color they are supposed to have. They also were covered in blooms.
This Plum Delight loropetalum is twelve years old and has endured some harsh winters in my zone seven. I have trimmed it several times to maintain control of it's size. There are very few of these plants in my area although they are recommended for planting by our local extension service. I have never fertilized the plant.
Check out this site. It may be of some help to you. Mine bloom vigorously every year and I do NOTHING to them other than light prunning in the spring after it blooms.