I had the same on my LOTR.
I had to loosen the metal rail (under which the left edge of the Cliffy goes) a bit which is located under the ramp.
To do that you need to loosen the nuts under the playfield, holding it (You should have had to remove or loosen them, to install the Cliffy protector).
I guess it is just a millimeter higher with the Cliffy which causes the ball to get stuck.
However as I loosened the nuts a bit, the ball still was able to go around the ramp without getting stuck.
I hope that works for you as well as it did for me.
Please don't forget turning the game off before servicing!
If you have further questions, be sure to ask! 
Newton, NJ
