Here's another view of the grip. Make sure the bones curve away from each other as shown. Press the middle bone firmly into the crease of the palm (the lifeline). It's tricky keeping pressure in the middle finger while keeping the ring finger relaxed. Try pulling the ring finger slightly up and back to keep the moving bone away from the palm so it can swing freely. Make sure the bones aren't touching each other, and that they aren't too far apart. Make sure that not too much of the bone is sticking out above your fingers.

You can also try a variant of the grip with both the middle and index finger between the bones. This works better for some people, especially those with smaller hands.