Did you reflow solder on all pins yet? That would be step #1 before considering a faulty component.

From pinwiki:

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Yes it's a common issue that happens with the Beckman glass panels used on the Stern displays. Your can try to raise the high voltage (within reason) which might wake up that particular segment.

While there are several good explanations for a segment being out on ALL digits, If it's an issue with a single segment on just ONE of the digits most likely it's a glass panel issue.

I had a display with a segment issue that actually killing the master display parts board over and over. After the 2nd time a segment resistor burned open and the logic level to high voltage level chip fried I realized the problem was in the glass itself. New glass, no problem since. Never seen that happen in a bally game, but its been posted about a few times here in WMS games. I couldn't really see any issue with the glass itself, but it must have been to much current to drive that segment.

I have had a couple of display glass with resistive crud shorts between the pins just after the pins are glued but before the pins enter inside the glass. The crud is hard to get to but can be removed with a small shaped wire/needle to bring them back to working order. Your multi-meter can show the shorts, typically the resistance between good adjacent display pins is around 600k ohms. Pity I don't have a picture handy showing the crud shorts.

Sometimes the NOS glass are more expensive than a working good used complete display.

Depending on what digit it is (or how bad it annoys you), you should be able to use that display as the credit/ball display, and if that one is good, swap it into a score display spot.

as it fits that topic and might help someone:

I had a display where the ´dot´segment and the lower horizontal (d) segment would always light together although they should not
The pins for those segments are #10 and #14 - checked for some shorts between those pins outside of the glass but there were none.
Resistance measured was around 70kOhms which was still enough to make them light together-so there was an internal ´short´inside the glass

As there was not much to loose I tried to burn away the internal short-applied 30VDC between the 2 pins. There was a small current in the range of some mA running for a couple of seconds and then stopped.
checked the resistance between the pins-went up to the MOhms range
Resoldered the pins-tested any everything is fine now - no more internal short and every segment working as intended

...sometimes one gets lucky and things work out as intended...

Well done, I was originally thinking of doing similar but with hundreds of volts.

I had exact the same thought but the 50kV HV generator I had at hand seemed a bit too much
I already wired the 4x 30VDC lab supply in series to get 120Vdc but in the end 30VDC did the trick