i am having the exact same issue, as we speak...
i disassembled the entire mech, cleaned it thouroughly, aligned it, everything you can think of. i even had several fellow pinball collectors look at with me, and we're all scratching our heads. it refuses to make it into the goal.
mine was even going as far as to bind completely, until i took lloyd's advice and threw a drop of thin lube between the cam, and the mounting bracket.. now at least it doesn't require me to shut off the game to retrieve the ball.
i personally like the stronger coil idea. it's not ideal, but in my case, i'm out of other ideas.
of course any advice from others is always welcome