The weather conditions. Oleanders like hot, dry weather and like to be in full sun in rich soil. Often, in Britain, particularly if we've had a cold spring (and we did, right up to mid June) the buds form too late, just as the days are shortening and the temperatures falling. Sometimes, in a mild winter, they may remain on the plant and open early in the spring, but mostly they don't. If yours are in full sun in a southfacing position and the weather continues warm with sun, they may yet open.

I nurtured an oleander from a cutting some years ago, I think it flowered once, in one of the hot summers (but not 1976). It is a Mediterranean plant and the cool, dull start to this summer probably wouldn't have encouraged flowering. That said, I saw one at the weekend in a nursery, but it was growing under cover and was in the 'houseplant' section.