sorry, i dont really look through - are we talking about high voltage fluorescent lighting? or about standard fluorescent tubes?
if High voltage: Do very careful work at their connections. Loose connection can still make the tube work, even if there is for example a milimeter distance between wire and terminal, depending on the voltage the tube could still ignite and work, but with an arc burning at the loose connection!
I have some HV tubing light up our terace, when i havent run it for a longer time or/and its cold outside, some tubes show this scenario. I believe that something happens with the mercury / merc vapor, so when longer running the merc gets vaporized again what makes the light brighter.
if talking about standard fluo tubes: I've seen the dark-in-the-middle problem when running new, cold tubes on Electronic ballasts. Conventional coil ballasts should only make the tube flicker a bit and brighten up delayed when its cold.