From the rule set (on pinball.org)

THEATRE MULTIBALL
The lock is initiated by hitting either ramp while the 'Lite Magic' indicator is lit (which is lit through ramp shots), or through the secret ball lock. Secret ball lock is accomplished by the ball not quite making the orbit. All balls are ejected to the right flipper from the Basement (its a three ball multi-ball, however if you make a ball Vanish, and do not collect the Tiger Saw Multiball, it will be a four-ball. You will then have to Vanish a ball afterwords to re-light TSMB.). Jackpot is scored by hitting the trap in the Trunk wall (which is the side now facing you). While the balls are being ejected initially the lovely assistant will count 3-2-1 (rather quietly, you may not be able to hear her in a loud environment). A jackpot shot during this will give you 100 million points. Three shots to the trunk re-lights jackpot. Ends when less than two balls are in play.

I have had problems with the vanish saucer kicking the ball out instead of capturing it (after the machine says "vanish" and awards points). But I made an adjustment to the arms on the vanish switches, and haven't seen it come back.

But this is different, and 100% reproduceable in two variations:

1) Light the locks, and before locking the second ball in the subway, "vanish" a ball. The ball stays locked in the vanish saucer. Then lock the balls in the subway. Upon locking the 2nd ball, instead of getting to launch a new ball, it kicks one out of the trap door.

2) Lock two balls in the subway, and before starting multiball, "vanish" a ball. The vanished ball kicks out immediately after awarding points. After multiball, the vanish is lit again (as it should be).

2a) Also, if I shoot the right orbit, it starts Tiger Saw multiball, but there is no ball in the vanish saucer. So I play one ball Tiger Saw (which kinda sucks because I can't start any modes, or start Theatre multiball when in Tiger Saw multiball). If I keep the ball alive long enough, it does figure out that I only have one ball, and returns me to single ball play.

Basically, this means I can never get a 4 ball multiball.

Has anybody experienced this? I believe this worked properly at some point in the past, on occasion.

Interesting problem.

First of all, when you open up your machine, how do you get the balls out? Do you run the ball eject test? Do you flip up the table and get the balls as they fall out of the trough? Do you pull them out with a magnet? Or do you just add a stopper so they don't come out? On my Sega table, I do none of the above. Sega has a mechanism that prevents the ball from falling out when the table is flipped up.

The solution revealed itself when I started suspecting the trough opto switch. The switch test indicated everything was fine. But in the process, I ran the Ball Eject test and viola, all but the last ball ejected.

It turns out, the plunger stopper was thick enough that it caused an TINY obstruction. As long as there were 3 or 4 balls in the trough, everything worked fine. But when there were only 2 balls in the trough, when the ball was ejected, the last remaining ball would be obstructed and remain between the last two optos. This caused the software think there were no balls left in the trough, and eject the ball from where it was attempting to be locked.

So the solution wasn't related to the lock switches, or subway.

TOM_trough obstruction - marked up (resized).png

Perfect.

I adjusted the two screws to shift the coil down as far as it would go and no luck. I didn't even realize that the proper way to adjust it was the 4 screws. Thx.

That's why I post these things.