Nixie tube failure?

[ITman496]

So I made a clock a while back, and it's been working great, except that one of the digits seems to be acting weird..

I am ignoring that my drive system could be causing it since if it was, surely the other digits would also be doing this?

It appears to look like cathode poisoning, but I'm not so sure.  It happened pretty suddenly (over the course of a few weeks) across all the digits of the display (and it was on the seconds display which is the most constantly changing display in the clock)

I'm very confused.  Is it just a defect?

[lowimpedance]

I does look to be a dud tube rather than the drive. Next question is do you have a replacement and is it easy to remove ?.
Substitution is the quickest way to test both the driver and if the previous tube was a dud.

[Gyro]

If it's cathode poisoning then it's sometimes possible to recover it using brief application of higher than spec current to sputter the surface clean. Try googling it. eg.

http://www.tube-tester.com/sites/nixie/different/cathode%20poisoning/cathode-poisoning.htm

The other possibility is that some Soviet (can tell it is by the upside down '2' for the '5') Nixies can suffer from physical cathode erosion due to absence of mercury in the fill mix. It's worth checking the affected cathode with a magnifier, and look up the exact part number for that problem.

Edit: The paragraph above the photo:
http://danyk.cz/digitrony_en.html