Cold Cathode Lights

[Preditor631]

A few months ago i bought a set of cold cathodes to mount on my desk (the desk was my Design & Technology: major design project for yr 12). i mounted a 145W psu (from an old computer) to the desk and did a custom wiring job on it to extend ONLY the 4-pin molex connector.
I then used all the normal wiring to connect the lighs, then glued them horizontally to the underside of part of the desk.

So my problem is that one of the lights now only lights up half the lenght of the tube, it just fades to nothing at the end. I found that if i flick or tap the tube that it will for that split second light up the full length.

Note: when i first found this problem, i had not used the lights for a week or so, but i had left the PSU turned on( only light switch turned off).

Any suggestions as to why this might have happened? thanks


Edit: I think i have probably just found the source of the problem. When turned on if i push on the problem tube in a certain direction i can hear the sound of air movement. but not when it is turned off.
Why might this happen?

[Random]

This could be one of two things, either the gas in the tube has "expired" and you need to replace it or there isn't enough voltage getting down the tube to illuminate the whole of it, in which case you need to check your wiring.