From your description, it is possible that the helium has migrated out
of the tube over time, and now will no longer sustain lasing at the
voltages the PS is putting out. Helium can leak through anything (it is
used as the definitive leak detector) and usually when a HeNe laser gets
cranky, it's becuase all the He has snuck out, leaving the poor Ne to try
and do the work of two atoms... c'est la vie....
Cheers
John