Answer: There must be a notable difference in growing conditions for the oleanders in the center compared to where the ends of the hedge are growing. Oleanders will grow differently in shadier vs. sunnier spots, if the soil moisture is inconsistent (due to irrigation system performance or surface drainage patterns for example), if the soil itself is different (as can happen with new construction where topsoil may have been displaced), or if the planting technique was inconsistent (such as encircling roots were not cut or directed outward at planting, plants were set too deeply, etc.) or if fertilization is different (lawn fertilizer may be reaching the roots of some but not others). It will take some detective work to determine what is happening. You may want to consult with your local county extension and/or professionally trained nursery staff to try to determine why the center plants are not doing as well.
Creeping gardening would do better in morning-only sun rather than hot afternoon-only sun. Your local professional nursery staff should be able to help you identify plants that would thrive in the planting spot you have in mind and also meet your overall design criteria.
Q&A Library Searching Tips