flatworms

[i have crabs]

ive had a small population of flatworms since buying some rock in january in one of my tanks, the first time i used flatworm exit most of them died like they should have, a couple days later i still had a few so i dosed it again and only a few more died, the third time i dosed about double the ammount and it didnt do squat.
i left it for a couple months and the worms have increased in numbers but still a very small amount, ive added a mystery wrasse and i havnt seen him touch them at all, just a couple days ago a dosed flatworm exit again, right off the bat i did douple the ammout about 160g worth in 60-80g or water i left it overnight and dosed another double dose in the morning so a whole bottle in less than 24h maybe even less that 12h actually, it killed some worms it killed my pods but it didnt kill the flatworms,
i would swear flatworm exit is garbage if it didnt work so well the very first dose, so im thinking the rest i have are resistant to it.its a very small population right now and hopefully will stay that way, i might try some other fish to eat them but havnt decided yet