If the game works, it's repaired. Call me a hack if you want.

Keep in mind as some of the nylon parts are over 40 and even 50 years if age, they seem to be expanding ever so slightly in size.
In the VFW collection, we've had four Bally stepper unit gear assemblies expand enough to no longer freely rotate in the sleeve so we've replaced them and even spun some down.
On some Gottlieb Decagon units, we've purposely left the R-22washers off to eliminate some of the binding you mentioned.

Do you polish and lube the circuit board?

Personally I think the decagon units are just a bad design at some level...

The problem is with the wireform that rides on the score reel itself and moves to change the switches. When the reel turns over to 9 it closes the 9 position switch. That switch is usually adjusted too close to the wireform, and when the wireform moves it puts too much pressure and the reel and it hangs up. This is probably the most common problem with decagon reels.

Cutting loops off the return spring is not really that likely to be fixing your problem because it's not the pressure of the spring keeping the reel from stepping, it's the switch adjustment. It's the return stroke of the plunger that actually steps the reel so you want a strong return spring. You want to adjust that 9 position switch so that it just closes when the reel steps to 9. That will fix your issue.

The PCB has nothing to do with it either. The reels without a PCB have extra spacers installed so that the pressure on the wireform is equal to what it is when there is a board there.

They're a terrible design. Overly complicated. Bally and Williams reels of the era are much simpler and much more reliable than a decagon.

Shortening the return spring is something he mentioned when the reel overall is sluggish and really should be taken apart and cleaned.. Here is what he says about the 9 position problem

"Clean all the nine and zero position switch with a flexstone. And make sure they operate with a good wiping motion, and adjust accordingly. But be careful in adjusting the zero and nine position switches. There is a balance between switch blade tension and the amount of "horsepower" available to turn the score reel. If the switch blades have too much tension, the score reel may "hang" and not move past the nine or zero positions. This is a common problem, and some (incorrectly) change the return spring tension to try and compensate for it."

I seem to remember that there is a spacer washer that goes on the spindle. If missing or in the wrong place the reel can bind when it meets resistance. You would likely see it listed in a parts list picture.

Admittedly a sucky design. The last version is the best however, as it is designed with no metal to metal contact. The nylon switch lever and ladder are also a big improvement.
On the earlier ones, I just thoroughly clean, polish and lube everything really well, and endlessly fiddle with the switch gaps and tension, until it works right. I shorten the return spring only as a last resort.

Allotta times the tension on the switch blades is too great and makes it too much work for that actuating arm to push them as far as they need to go when in the 9 position, and it trying to get to zero. Make sure that switch tension isn't too great before hacking the return springs..I often get it too tight while adjusting and cleaning that switch stack.

Like others have pointed out, your problem lies in the carryover switch adjustment. Keep in mind that lateral movement of the score reel on the spindle needs to be kept to a minimum or the switches won't operate correctly.

Here's what I discovered on my El Dorado - admittedly, my only GTB EM and a late one. But in going from no score reels that worked when I got it, I was able to get 3 of them reasonably reliable with basic cleaning and tuning like others have suggested. So it stood to reason the 4th should be no problem following the same procedure, right?

Well for some reason, it just wouldn't work reliably like the others. Which leaves that "hack" of adjusting the spring...

...and honestly, is that really a hack? We concede that other parts wear out over time. And if some consider that nylon parts swell and/or shrink and wear over time... well it seems far more likely to me that comparatively flimsy, thin metal wire coiled into a spring, may have itself stretched and/or oxidized and/or bound in ways to weaken over time.

If you get into a position like mine, you can swap springs among reels to see what happens. If the problem follows the spring there's no shame in modifying or replacing it. I've done both, but replacement is better since there's no way to be sure how long a "modified" spring may last.

Usually you only have to shorten it by a single loop or two. Be warned, if you go to far (make the spring too short) you can make the reel stick elsewhere. Even if it works, if you hear *any* buzzing when the numbers change, the spring is too short and making the coil work too hard.

Removing a coil or two from the spring won't hurt anything. Put a drop or two of light oil on the spring, flex it a couple of times, then wipe it off. It'll keep the spring working smoothly and will retard rust and corrosion.