how long did you let it run? It has to warm up the blue in the arc tube haves to heat up the gasses before it will ignite. It should only take a minute. If that don't work check the socket make sure the contact tab is not smashed down to much if you do that its a real good idea to unplug it before you mess with it. When I buy a new bulb from the grow store they score the bottom of the bulb that could be it to? Did you make sure its switched to mh and not hps?