Can the WBT's act as a resistor?
The solder joints could, if not done properly, and the contact with the cable terminations could, if oxidation, grime, etc. is present.
But my instinct, not having seen the work being performed, would be that the number one suspect would be the possibility of mechanical jostling affecting the tubes. Perhaps by dislodging minute detritus that may have been present on some of the elements. Especially, as I said in my previous post, if they have been heavily used.
Regards,
-- Al