I found these tips years ago and it works perfectly. Warm the milk/mixture if adding yogurt too, slightly before mixing in.
Also very important, line and grease the baking tray. Heat up the baking tray whilst making the scones. Do not remove from the over until the scones are ready to be baked.
As soon as the scones are ready to be baked, remove the baking tray from the oven, place the scones on the tray, quickly brush the tops with milk/egg and put them straight back in the oven.
Its the heat the scones need. They rise perfectly every time now