Quoted from x-rug-x:

My fix to making the loop consistently
Like many, perhaps all users, my pin was able to make the loop 100% out of the box with both the auto-launch and the manual. Then mysteriously over time (about 4 weeks) the auto-launch would miss 1/20,1/10 and eventually 19/20, but for whatever reason manual would still work. It was consistently hitting the post right below the loop (low). I was sure the auto-launch mech was to blame and I focused on that. I eventually made it so both the auto-launch and the manual would hit low (but I never touched the manual launch). but as it turns out the auto-launch wasn't the problem.
I saw other guides that mentioned bending the right-side guide rail to solve this problem, but I really didn't want to do that (I didn't like the idea of bending something that didn't seem like it needed bending). Plus, it seemed like this solution was susceptible to wax, cleanings, or even strange looks. Regardless, after exhausting all possible adjustments to the manual/auto-launch mech, I started down the bending the guide path. What I noticed was that as I removed the end screw the guide, the end lifted up and became more vertical (closer to perpendicular to the playfield). I tried a shot with the rail in this position, made the loop instantly. Tried a few more. Perfect, just like day 1. I found a washer that was the size of the gap and tightened it down. It hasn't missed a shot yet. The guide is not 100% perpendicular, but it is the natural position the rail wants to be in as opposed to being torqued down.
Simulated view looking at the far end of the right-side rail.
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This might require re-adjustment too over time, it's too early to tell. Good luck, and let me know if this works for anyone else!

Quoted from Gogdog:

As I understand it, the washer would go under the guide wall tab. I.e., it would be PF, washer, guide tab, screw.
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However...I think this would only be a fix for those that have a guide wall that is torqued. I.e., where the guide wall is not perfectly perpendicular with the playfield. I'll go ahead and still add it to the plunger adjuster guide here ->
https://pinside.com/pinball/forum/topic/iron-maiden-issues/page/25#post-4645095