Many gardeners think of oleanders as no-care plants, because the shrubs are often used in road medians and other harsh areas. They do require some care, however. First be sure that yours are planted in the right spot; it sounds to me as though they're not. Oleanders do best in full sun; if yours are in partial sun, they may fail to thrive because they're getting too little light. Transplanting them to a sunnier spot may solve the problem. In most average soils, oleanders do well without fertilizing. If your yard has especially poor soil (a soil test can help confirm this for you), the plants might benefit from an application of timed-release fertilizer in spring. Follow the directions on the package to avoid overfertilizing your plants. Overfertilizing can be just as harmful to the plants' health as getting too few nutrients.