I can't see a forever fix tbh. I had a Skateball that had issues like this. If you look at the layout of Skateball it shoots across the playfield into a loop and up into the rollover lanes on a full plunge.
Now, if the playfield is dirty, clean, plunger doesn't hit it quite right, etc. it won't make that loop 100% of the time. It's just a fact of mathematical angles/physics/whatever you want to call it.
You will be able to get it close, but it will never be 100% imo.