You're seeing the same phenomenon that the medical community has with antibiotics. Thrips are going to develop immunity to any systemic eventually. The more effective methods are the old-fashioned mechanical ones like soaps and oils, jmho.
I've been fighting a similar battle with spider mites on brugmansia the last few years, and my neighbor finally convinced me. He was caring for my brugs while I was away on a trip, and they were looking pretty sketchy when I left. Ten days later when I returned, they had rebounded amazingly. When I asked him what he did, he said he just gave them two really thorough baths with soapy water using a 2-gallon sprayer.
Btw, that same neighbor has his Masters in Hort from UF and worked as a pesticide inspector for the State of Florida for some years.
Cross posted with Anne, and yes, DE is another great 'mechanical' solution. Especially good for indoors because of no smell, and also not impacting beneficial insects as it can outdoors.
