I don't own a Isonic, but they are not rare. I sailed a few spread over the last years. It could be your sailing style requires more lift from the fin. There are 2 ways... Longer fin, but you will loose control/speed in higher wind
or just go for more flex in the fin. If you look @ select fins these models have more taillift the Select S07 softness Medium /ghost/ proslam.
On my CA speedboard (also v-bottom) it took a while before I was able to sail good speed with a standard SL7. I really prefer to sail a Select S07 M underneath my CA Speed.