If your running the standard Coreban fin set , sounds to me your are trying to surf from the middle of the board , that's why it's hard to turn and then when it does turn it can slide because you don't have enough weight over your back foot. Weight on your back foot not only helps the board turn but it also keeps pressure on the fins and rail to hold in the turn.
I assume you have your side fins in ? place your centre fin in the just forward of middle in the fin box , and surf with your Futures fin key in you pocket as it will fit the screw on your Coreban fin box . Catch a couple of waves and move the fin back 10 to 15mm. You only need to loosen the screw a few turns and you can bump the fin back.
Keep doing this until you find the sweet spot , remember to move your feet taking pressure on and off the fins and as you start to lay the board over move start laying on the paddle , you will be chucking buckets before you know it.
Check out this pic of Ivan below , I shot this a few yeras ago when he came to Noosa..... check the board on it's rail , feet position and pressure on the paddle.
That's what's it's all about
