Those CCFL requires a very high voltage to ignite. Often the moisture lowers the start voltage by affecting the flyback transformer or some capacitors, or the transistor goes off specs.
I found that attaching with some glue a thin aluminium foil under the CCFL tube, (on the plastic "reflector") helps a lot, because it increase the startup voltage gradient IN the tube.
Alternatively, you can "hack" the ballast, by decreasing the value of the biasing resistor on the base of the oscillator transistor.
Hope it helps
Anthony