Why are scales so much harder to treat than other insects?
It's because many of them cover themselves with a protective waxy coating.
Indian Wax Scales illustrate this topic very well. They are easy to identify because they look like chewed gum and are usually found on the stems of plants like Gardenia, Magnolia and Holly. Adults are completely immobile (they don't have legs) and simply attach themselves to stems or leaves like a tick on skin. For the duration of their lives, they feed on plant sap and reproduce (how sad). Since they can't run and hide like other insects, they protect themselves from predators and the environment by secreting a waxy coat.
This image is of the underside of a Wax Scale that I picked off a stem. The actual insect is dark pink and the waxy secretion is white:
The wax is easily separated from the body of the scale and is the consistency of peanut butter. The soft-bodied insect within is very vulnerable and cannot survive without this coating. This coating prevents insecticides from reaching the insect inside. Next time, try horticultural soap. If applied correctly, soap can break down the waxy barrier and dry out the insect inside.