Quoted from AntennaMan:
I thought I had a trouble free IMDN, then I read this. I never knew the ball was supposed to go around the loop, either via auto or manual launch, it never occurred this was how it was supposed to be. Now I realize I've had a broken IMDN since day one.
Yes, I remember reading somewhere that when the game was originally designed, Keith had not really planned on the ball launch going cross-playfield, although the end design ended up showing that it went around that left loop rather consistently. It was upon this discovery that they started developing some code that encompassed that, i.e. loading balls for trooper multi-ball. He (Keith Elwin) says on later interviews, I think at the time when he finished making Jurassic Park, that one thing he learned from his first game (Iron Maiden), was that he was going to no longer have any games that encompassed cross-play field launches, (my guess is because it tended to be unreliable at times, as several have seen).
But yes, upon full launch, it should go directly into that inner loop, all the way around and then back down to your right upper flipper. Auto launch should ideally go in that same pathway. During The initiation of trooper multi-ball, the ball should go in that pathway and get stopped by the post and load up just behind the upper spinner, normally two balls from auto launch are kicked out so that three balls (third ball being the ball that was just locked) are released upon the beginning of that multi-ball. The balls then all come down to the left upper flipper at the start of trooper multiball. You'll notice that if any of those two auto launch balls don't make it into that loop, the game recognizes it and immediately initiates the trooper multi-ball (recognizing that auto launch failed to make the loop). So, it was obvious at the time of code development that this auto launch to loop shot was likely, but not a guarantee. Personally, I love it when the balls make it up the loop in prep for trooper multi-ball because there's a pause and then a call-out ("The charge begins!") that starts the multi-ball and is rather exciting
It appears as though the one of the main culprits for not making the auto launch is small burrs on the very edge of the metal on the ball guide. At the end of the launch ball guide, right there at the gate, if you look at that edge of the ball guide that faces you, it can get hit by balls during play and small burrs are created. These small burrs can/will deflect the ball ever so slightly lower on its trajectory, thus increasing the likelihood that the ball would hit that post in between the loop and the skill shot target as opposed to going into the loop itself. Filing off those burrs would be an option in this scenario.