It also is true that many insects have varying susceptibilities to insecticide active ingredients. Our most potent example these days is The Common Bed Bug, which, as one of our industry researchers put it recently, has an amazing ability to develop resistance to chemicals in general. Our most important example of an insecticide that can be metabolized to molecules that are not toxic to it and then to excrete the materials is pyrethrum. It has long been known that the German cockroach is hard to kill with just pyrethrum alone, and this is why piperonyl butoxide is nearly always added as a synergist. The roach can break down the pyrethrum molecule, so the PBO in some way blocks that ability and together the two have a better chance of killing the roach, and many other insects. With pyrethrum alone the German roach may flop over on its back and look dead, but commonly they will overcome that knock down effect and survive to pester us another day. 

We should not confuse "resistance" with "immunity". The roaches and bed bugs are not immune to any insecticide molecules, but through natural selection and exposing these insects to certain active ingredients or groups of related active ingredients for many years, we have created the monster ourselves. Those individuals that had a natural ability to withstand the "normal" dose of the a.i. may have survived and passed that level of resistance onto their offspring. Since insect breed so rapidly and in such numbers we can witness that resistance ourselves in just a few years of using those insecticides. 

What is needed to kill any arthropod pest with a chemical is the combination of Contact Time and Dose. We can still kill bed bugs with products they are resistant to by confining them to a treated surface for a longer period of time (sometimes a LOT longer), or we can increase the dosage rate of the active ingredient and kill them in less time. This is fine in a lab, but in the field we cannot continue to increase the dose above the Label rates. So, we must place the active ingredient onto the surfaces where we expect the pest insect to spend the most time, and for roaches and bed bugs that is going to be directly into the crevices, holes, gaps, cracks, and voids where they spend nearly 18 hours each day. As long as they are not repelled away from that treated surface by the nature of the active ingredient the bug now is resting on a contact insecticide for 18 hours, slowly absorbing it through the unbroken cuticle and into their nervous system. 

So yes, in a sense you are correct, but the missing ingredient is "placement". We can use the right product and the right equipment but apply the material to surfaces where it doesn't have a chance to contact the pest arthropod for a long enough time to get enough a.i. into it to kill it. A nice added benefit to crack, crevice, and void treatments is that not only are they the best places to kill the insect, but they also are the best places to avoid unnecessary contact by people.