Gday Hilly,
We haven't had any problems turning the Mano's regardless of wave size, we ride them from grovel up to 3-4 times overhead reef surf.
Two things that come to mind if you are having trouble is where your feet are on the board and fin positioning.
I find that the Mano's performance based shape require you to be back and surfing off the tail - for example the JL Hanalei allows you to commit to turns from almost anywhere on the board - whereas the Mano's have a more defined sweet spot - with the back foot being back near the tail pad kicker region. I am right on the kicker in small surf.
Personally I have to not go all the way back to the kicker wit my back foot in double overhead + surf as I find the board is too loose and turns too quick from there for my liking in those kind of waves. I tend to be roughly 6" from the kicker in these waves.
My centre fin positioning for up to head high waves, is right up against the top of the box. Double overhead surf I have it back about 1"- 1 1/2", then after that it is in the middle of the fin box.
Here's a couple of pics on the 9'4", centre fin in these shots is 1" from the top of the fin box.
Cheers,
Courts