If you spray houseplants twice yearly or so to rid them of scale to no avail, you're not doing anything wrong - just not enough of what's right.
Mature scale insects can be extremely frustrating because they're protected by a shell-like cover. But when this is removed, the insect dies. So, one effective method to get rid of them is to use your fingernail and flick them off the plant (or use a stiff brush if you're squeamish). Young scale insects, called crawlers, are less easily seen but are easily wiped away.In any case, a soapy spray is not as effective as a soapy bath. The real secret to eventual success is to bathe plants repeatedly, at least once a week. This is because the insects reproduce par-then-o-gen-ic-ally: even if only one survives, it can reproduce all by itself.
To make an effective bath, fill a sink with a gallon or two of warm water. Add the insecticidal soap, following the package directions if it is concentrated. If it is the prediluted mix, remove the spray top and pour in about a third of a cup for each gallon. Turn small plants upside down and hold the stems under the water for several seconds. (Keep the soil in place by covering it with tin foil or with a wet paper towel, and hold your hand over it when you turn the plant over.) Large plants can be laid on their side and their limbs gently dipped into the sink.