Stop watering and let them dry out - they are VERY drought-tolerant. They are just resting and will bloom again after they are done resting. Also, spider mites LOVE this plant, so spray the foliage once in a while to discourage them.

The plant or the flowers? Oleanders are very drought tolerant water twice a week until established. I have the double and single pink growing with no care for several years.

Here is a link that might be useful: my rustic bajan garden