Hi Judith,
In my experience, the timing of spraying is critical. I use only white oil with 750g/100l of sodium bicarbonate and have had quite bad infestations of black scale. In the last couple of years, I have gained much better control by examining the scale under a microscope before I spray.

There's no point in spraying the adults as the oil can't penetrate their hard shell, which protects the eggs. You need to get them at the crawler stage to have much effect. If you turn the adult scale over, under a miscoscope or magnifying field glass, you should be able to see the coral-coloured eggs. If you tap the adults on to a white sheet of paper, the eggs will show up against the white background, without the aid of magnification.

Hold off your spraying until most of the eggs have hatched, leaving behind the fluffy white remains of their shells under the adult shell. You then have a period of a week or two in which to target the crawlers. You will have to repeat this exercise 2 or 3 times over the season. This year, the eggs were very slow to hatch, probably due to our mild start to summer in WA, so I saved a lot of time and money by waiting. I look on my purchase of a basic stereo microscope as one of my best investments yet. Apart from the obvious benefit, it's very interesting to look at the insects under good magnification.
At the very least, get yourself a good field eyeglass and carry it with you whenever you're in the grove.

I hope this is of some help.
Regards,
Julie