My back ramp shot needs to be pretty perfect, but not TOO bad. I can make it about 70% of the time. Other times, it rolls half way up, then comes back down.
I've had a similar issue on a Firepower where I couldn't make the upper lock from the right flipper. It turns out just lowering the angle of the flipper down a bit (just a hair towards the apron) made a HUGE difference, and now I can make that long, looping shot easily. Give it a try if nothing else pans out.