How do I get our oleanders to look healthy and full?

Have you given them any fertilizer,are the leaves browning,do you notice any insects on the bark or leaves?

Are your oleanders getting plenty of direct sunlight? If they are in a shady area or too near to your house they might not be getting enough sunlight.

If that is not the case consider pruning them after they finish flowering, cut them back by about 1/3. Also consider if your soil might be poor and lacking trace elements ... if this could be the case apply a good general fertiliser or dig in some animal manure. (Not chicken (unless you are an experienced gardener), as it is high in nitrogen. Dont forget to handle the prunings very carefully as all parts of the oleander are toxic.

For any specific plant, many things can affect its health. If your oleander is getting too much sun or shade, is getting rusty runoff from your house, has insects or their eggs eating into the core of the plant, has bigger animals digging around the roots and eating the branches, or is lonely (for pollen) since you only mention one oleander, your plant may need your intervention. Those really healthy plants you see may be stealing what moisture there is from the soil and affecting the foundation or sewer lines from their nearby buildings. Those oleander bushes may also be ten years old and older and are therefore well established. The most likely thing is, you may have spoiled your oleander by watering it regularly. It may be absorbing that but not sending out underground roots seeking water, so it's root ball is stunted. You may lose this plant anyway because of any one of the factors above. Why not try benign neglect? Leave the oleander alone for a week, water deeply, leave it alone for two weeks, and so on. You would be weaning it from depending on you and You can decide what it's future may be. Best wishes 😇