I seem to recall the old neon bulbs had some slight radioactivity introduced into the electrodes to reduce the starting voltage. As they age, that weakens and makes them less likely to start. They were typically fed from a very high value current limiting resistor, some of which were not all that stable over long spans of time, so there are several paths where age could be taking its toll. There used to be a booklet showing all sorts of clever tricks one could do using neons as a non-linear element. Fun little items slowly passing into history I guess.