You can certainly try a larger fin, maybe 3-4 cm longer, to use with your biggest sail. Other less expensive things to look at: the condition of your original fin, and how much you are pushing against the fin with your back foot.
The fin should be "clean" with no dings, nicks, raised print on the fin itself.
Sometimes when you change from one board to a new one with a slightly different design, especially at the tail end, that change will bring to light some techique issue that needs to be resolved. Too much pressure with the back foot, the position of the footstraps, etc.
hope this helps,