Take a break for a while and try Iodine again. A bug becomes vulnerable again in the same manner that it became resistant. Some of them mutate every few hours. When termites become resistant to a certain chemical, exterminators simply switch to a different one. A couple years alter they go back to using the original one. Are they changing the formula and calling it the same thing? Or are the termites no longer resistant because they haven't been exposed to it in a while?
You have to both exterminate AND strengthen your immune system, in order to keep the progress you've gained.