Horticultural oil not working on aphids

First of all.. before anyone mentions how the natural cycle of the ecosystem will control pests, it truly doesn’t where I am. These aphids are beyond out of control and must be at least 50 per terminal bud on my siberian yarrows and there are also plentiful on my azaleas. These are the green ones. The black ones infest my sedums every year. So 2 days ago I sprayed the living heck out of my yarrows and sedum(followed proper dose of oil, went a little over the recommended actually), first giving a good spray on the overall plant and then grabbing each stem and spraying at the terminal buds one by one. Today I check them the aphids are happy as ever. How did the horticultural oil not work? It worked for spider mites on my cast iron plant. At this point can I assume its safe to invest in one of those expensive systemic insecticides?

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