The most effective way to get rid of scale is to squash them by hand. Not a huge job on a small tree, but still takes some time.
Some plants have adverse reactions to some systemics...Systemics an take some time to work as it makes its way up the stem of the plant. Could take weeks before it's fully effective
Regular spray-on poison has to be applied at exactly the right moment when scale is vulnerable (in its larval "crawler" stage). If such sprays are applied to scale that is already hardened off in their shells (as is the case in the photo) it just rolls off of them harmlessly.