If you had luck with Kursuri Plus wormer, you were very lucky. It's nothing more than flubendazole and a buffer. There have been several articles in the veterinary journals in recent years about certain fluke populations becoming resistant to flubenbazole along with prazi and some of the organophosphtes. Those drugs have been on the market for so long, repeated treatments with them have allowed some flukes to develop resistance to them.
Oxidizers like PP and formalin are the only treatments flukes can't become resistant too.