Scale, when bad, is nearly impossible for me to completely get rid of. What happens is I think I'm making progress and they have simply hid better. Usually, best rule of thumb, return them or segregate them until completely clean. The problem though, usually only plants with something already wrong with them get bugs bad. The weaker the plant, the harder it is to save it from the bugs or trash. It is tough to send them back, but if the vendor is willing you will be saving yourself months of agony, as well as alot of money trying to kill the little buggies.