Those scale insects can be tough -- we have them in abundance here on the gulf coast where it is warm and humid. To defeat a severe infestation try mixing some malathion with Ultra-Fine (a water soluble paraffin wax) which is available at any garden center. Spray the underside of the affected leaves thoroughly, and repeat every two weeks until the infestation is gone. Ultra Fine is different from horticultural oil, it can be sprayed even during the warm growing season (whereas the oil can only be used in dormant season). Once the infestation is cleared up, a regular spray of Ultra-Fine only should generally keep them in check in the future. Good luck!