At least once every half dozen or so games, the ball in play becomes lodged on top of the diverter arm. I think this happen if the ball hits the diverter arm when it is opening/closing as I find it stuck in the halfway position with the ball neatly sitting on top of the arm and jammed between the arm and the structure above it.
As I'm not getting any extra points for this "skill shot", I guess its not meant to happen.
Does anyone else experience this or have any ideas what going wrong?

If the diverter is stuck half way of the open/close position, check the spring that is used with many diverters.
When this spring breaks, your diverter will not fully open or close all the time.
Just replace the spring and your diverter will be fine again.

Remove the playfield glass, lift the playfield up, look at the diverter and check by hand if the spring between the plunjers and and the diverter arm is broken.

This problem about balls getting stuck has not happened on my machines so far.

Thanks Revo I'll check it out.