Bed bugs have good adaptability skills, and thus, they can and have easily developed resistance to insecticides meant to exterminate them.
This article linked below points out that even the insecticides meant for bed bugs who developed resistance to pyrethroid, a chemical traditionally used to exterminate bed bugs after DDT was banned,are becoming less effective due to bed bugs developing resistance to this. Some bed bugs are even resistant to DDT now anyway.
What spray can be used for bed bugs given that many are resistant to insecticides?
At the time I am writing this post, EcoRaider is the best performer of essential oil-based pesticides which overcomes the resistance-factor and has residual effectiveness (will work even on EcoRaider that’s been dry for 14 days). The article linked below has information about EcoRaider.
A good contact spray, which exterminates bed bugs on contact but has no residual effect, is SteriFab.