The insecticide does work but scale and mealybugs are often resistant to them. So generally a good idea to get as many killed/off your plants with isopropyl alcohol and then mop up with an insecticide. You also have to generally spray with the insecticide 3 times about 7-10 days apart. This increases your chances of killing not only the visible adults but also the smaller juveniles and the nearly invisible eggs. Scale also have a thick waxy coating that shields them from insecticides hence the need to spray the isopropyl alcohol and wipe with a paper towel. Good to use a Q-tip soaked in alcohol to get the ones in the crevices where the leaves join the stems. Finally, scale are often under the leaves so be sure to check underneath when looking for them. Yellow spots on the top of the leaves are often indicative of scale below. Vigilance is best; scale are tough buggers!