Ive got a 400w HTG grow light (magnetic ballast) with their grow bright 400w mh conversion bulb. The setup was working when I initially got it but I decided to try to fire it up again recently and I can hear the ballast fire up but the bulb is just a dim blue around the center, doesn't get any brighter even after about 10 minutes of use. Bad bulb?
How many hours did you have on the bulb before dusting off the cobwebs ans firing her up again? Some bulbs will simply fail early on. It does sound like the way an aged bulb acts when it fails due to age. One other possibility is if it was a MH bulb made to run in a horizontal or vert configuration and you had been running it in an opposite mount. Some Halides will quickly dim out and not burn at all when not mounted in the right direction, but some will run but severely shorten the bulb's life. HPS bulbs can run horizontally or vertically without a problem, but MH's are a different story.
I think the bulb is dead, threw my hps in there and it fired right up. Even waited about an hour after I tested the hps and then threw back the mh in there and same thing...
Just FYI, other than cloning plants veg quite well and don't really stretch that much under HPS light spectrum. I haven't used a MH for years without a problem.