Make sure you keep your shoulders and hips perpendicular to the slope. Since you dont have a lot of experience riding fast yet, your body would instinctively lean back uphill for several different reasons (mainly fear, vestibular righting sense, guarding reaction). Imagine putting most of your weight on the rear foot (you may not realize this is what you're doing) then trying to turn, almost a guarantee your edge will catch. The faster you go, the more you should 'attack' the slope.
Same reason why learning on a rocker board is so much easier, because the rear edge will still have enough clearance from the ground to not catch.