Light output on these displays drops over time due to falling emission and reduction of phosphor efficiency. How quickly and how far this falls depends on the construction and operation of the VFD/display.

Emission may usually be restored by raising filament voltage.
33% filament voltage boost can rectify moderate fade, and 66% boost can rectify severe fade.
However, This can make the filaments somewhat VISIBLE in use, so trial & error for preferred results ;-)

Either way, if you need to drive things harder to see them, your really only delaying the day it finally dies, which might only be 6 months away...
MM