400 hps HELP!

my htg 400w hps ballast will buzz an light will flicker but never fire on. If i unscrew the bulb wait 4 seconds screw it in it works. Whats the problem bulb or ballast? Any help would be awesome, bulb is new about 2 months same with ballast.
+rep for help THANKS

sounds like the bulb - as the bulb is cheaper than a new ballast, why not buy a spare bulb to test this out?