Think one of the optos is not registering right.
Inspect the optos on the spiderramp, but suspect the optos on the VUK

Does it always work properly in single player?

I had a similar issue, turns out one of the diverter arm in the subway was off a bit, wasn't sending the ball to the right spot.

LTG helped me figure it out.

Otherwise, I'm guessing it's most likely on opto on one of the subways exit VUKs as Thor said.

Welcome to the STTNG troubleshooting club, it can be a headache!

Off topic- I've been to Goteborg, what a BEAUTIFUL city!!!

I was there in 89 or 90, Volvo sponsored my high school band to travel there and we performed for an exhibition of American Football- Minnesota Vikings vs Chicago Bears.

I would definitely check the wiring loom for a start, sounds suspiciously like broken wires, which the loom is prone to do in these machines!

Those looms tend to break 'in' the wire.

Going back and forth, the copper inside cracks.

Check those for continuity, and while doing so, wiggle them

On my machine the both cannons make one swing to awknowledge the mark-home position.
Swing out and back to rest.

My left cannon does not test on start up, but works perfectly in game, and in menu tests.

When I asked about it, I was told that many other people's STTNG don't test the left cannon either. I haven't worried about it since, but I guess it *could* be the loom! It works normally in game play so I ignore it on startup.

on my machine, only the right cannon moves on startup. i think it might actually be a software glitch, as i've never had a problem with my cannons during gameplay or manual tests, and the same thing seems to happen with most peoples' STTNGs. i wonder if ROM version makes a difference.

Hmph. you jinxed me. Read your post and felt like a game. On mine, both cannons look for home on power up. BUT now I suddenly have an issue where the left cannon loads the first time, but the second time the plunger stays out, no ball loads and the cannon keeps cycling until you hit the left flipper. This points to a faulty opto, a harness issue and/or a loose connection. I have some investigating to do.........in addition to the middle of game reset, but I'm pretty sure that's a connector issue from the power driver to the MPU (or power driver needs bridge/caps replaced but that's unlikely) stay tuned.........

You're assuming canons rotating at power up in normal, it's not. It depends on software version, where the cannons where on power down, where the balls are at power down, the closed switches at power down, among other things.

If the canons are working normally in game play, and you have no errors, there is no problem with the canons.

--
Rob Anthony
Pinball Classics
http://LockWhenLit.com
Quality Board Work - In Home Service
borygard at gmail dot com

It could just be the plunger getting stuck physically. Don't overlook the obvious. Mine also did that. I solved it by replacing the plunger and sleeve, but I think the physical sticking can actually just be solved with a little filing.

The left flipper might just create the jolt needed to unstick it. Only way to tell is to turn the game off when it is stuck!

Yeah, I checked plunger freedom first thing and that's not the problem. I agree with borygard about software re:position at power up/down but I always let the machine reset completely for a new game i.e. wait for it to re-spot all the balls for a new game before I power it off. He also stated that if the cannons work during play with no errors, the cannons aren't the problem, but they sometimes do NOT work properly in game and with no errors. So his advice is sound--still sounds like a connection/opto problem in my cannon(s).