It could be any number of things. You need to take it apart and clean it, especially pivot points, and then perhaps increase the tension on a spring or two. Don't start messing with the springs till you've cleaned it.

Start by removing old grease from the two drive arms on the ratchet side. It is easy enough to remove them for cleaning. Add a little grease when finished.
The issue with steppers is how fast they operate. If there is any sticky old grease the drive arms dont keep up. It may work manually, but not when the coil fires. Take a pic before you start.

Instead of disassembling you might get by by just lubricating the moving parts with Tri-Flow Teflon lubricant. Get it in a 2oz squeeze bottle at a bicycle shop and apply it judiciously to the pivot points. Work each pivot by hand and apply more lube and basically flush out the old lube with new lube until each pivot moves freely.

You have nothing to lose.

This is bad advice in my opinion.

These things run FINE dry...and in fact the reason it's all messed up is because someone lubed it up 30 years ago.

Applying astroglide "judiciously" to this unit is just gonna make things worst - or at best a lot messier.

He needs to clean all the old crud out, and get it so the pivot points are moving freely, and he'll be all set. There's no way on Earth lubing this up is gonna work, it's far too late for that. The old crap has to go.

He needs to find out which movement points are gummed up and clean those parts thoroughly, not just add a bunch of new gunk on top of the old gunk. I'm as lazy as the next guy but this ain't gonna work.

If the plungers are gummed up they definitely need to be disassembled and cleaned. I agree that these should operate dry. But the metal to metal pivot points require lube and are not intended to operate dry. This is where you can get by without disassembling.

Yes, disassembling the unit would be most thorough but I find you can competently service a stepper with a minimum of effort without full disassembly.

I have to agree with CrazyLevi here. I've been down this road, a clean stepper is a happy stepper, and works properly.
Not to step on MikeO's toes, but adding new lube at this point to flush old gunk will just add new gunk in a short time.

Disassemble → clean→lube→ reassemble→ = Happy camper.

To each his own. In my experience of 200+ EM games I've only had to tear one stepper down completely to get them to function reliably. And that was on a fifties game where the grease had hardened solid on the main shaft.

I regularly pull the coils, clean the plungers and replace the sleeves, and just apply fresh Teflon lube to the metal to metal pivot points and my games play reliably. Again, what does he have to lose? He can always go all the way with a full teardown if this doesn't work.

Yes,and with a toothbrush gets it done well.

This takes about 5 minutes to do:

First put a rag under the Match unit to catch a runaway nut and toothed washer just in case.
Disconnect the 2 long springs first then employ 2 → 3/8'' wrenches,one between the bakelite and metal bracket the other on the stud.
Loosen till a few threads hold the nut at the end and swap it for a magnetic screwdriver (test for magnetic strength first) and turn the stud out and slide the nut /washer out.
Roll of the spring on the clapper arm and the e-clip on the other pivot.
Clean with t-brush/alcohol. Reassemble in reverse.

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Once the parts are off and when you go to remove the E-clips or spring,it better to do it in a cardboard box as a precaution to avoid chasing them around the floor,until you have done a few and know what to expect.
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Good work.

The point isn't that you need to do a complete teardown - you didn't. But you are always gonna have to take SOMETHING apart to fix a gummed up unit like this. Hosing that thing down with lube wouldn't have worked.

I guess I do it backwards, but I don't ever want to take a stepper apart before I fix the problem (or identify the problem with 110% certainty). Get it working, even half-a**ed and then take it apart.

Here's my theory, you take it apart and you're introducing new variables. You get it back together and if there's a problem, you no longer know if it's the original problem or something you created by taking it apart. Only change one variable at a time, and the problem stays small (and hopefully simple). It's a lot easier to deal with "Gee, it was working before I took it apart" than it is dealing with "I took it apart and put it back together and it STILL doesn't work!"

I was watching a video (maybe it was one of Clay's, I forget) and they talked about cleaning and adjusting every switch as a matter of routine before powering up the machine. If it works for them, fine, but to me it seems like madness. At least 95% of those switches were perfectly capable of doing their job the way they were, why make random changes? Get it working, then clean and adjust.

Great to here. EM are made to work and to be repaired. unlike most things made today.
Its good to here you enjoyed the fix too.

Hit it with a quick shot of wd40 and wipe it clean.

Well, there is some fine advise.

Hey, I'm with cjmiller on this. Every time I take a stepper or a reel apart, I introduce new variables into the problem. It is more often than not that I fix the problem only to have new issues arise. If I can fix the problem without taking anything apart, thats what I do. I am not using wd40 as a lube, it is a cleaner and degreaser. It can remove 30 years worth of grease in about 3 seconds. I then wipe all of it away (it is flammable, you know) and have a clean unit like the day it was born. No disassembly required.

I know there are better products for this, but I always have a can of wd40 around, so that's what I use.