Most standard hps bulbs only give out more to the red side of the spectrum which the plants craving for during thier flowering stage, have you got a multi purpose hps bulb? if not then stick with the flourocent bulbs for veg for the mean time. only light i got is a 400watt hps like yourself bulb i got the multi purpose bulb but they seems to react more to natural sunlight...i lil confused myself

Heres a pic of the package my multi-purpose bulb came in...hope this helps!