Hey there. I got a 400 watt metal halide lamp with an hps conversion bulb. Or maybe it was an hps with a metal halide conversion bulb. Can't remember now.
Anyways, I found that if I turn the light off, then turn it back on within 5 minutes, the hps bulb won't light up most of the time. Sometimes it will, but only after 10+ minutes of being turned on. Seriously, I will look directly at the bulb, and there is a tiny spot inside it that is glowing, but it takes at least 10 minutes to even start lighting up the rest of the way, when it lights up at all.
Works flawlessly when left uninterrupted on the timer.
May sound dumb, but maybe make sure that the ballast hasn't been turned on and off in the last 20 minutes or so. Then try the bulb. I doubt that that's the problem, but if you haven't checked, maybe you ought to.
I can't speak much on flowering with mh, but I've read of others flowering their plants with them, reporting looser, flufflier buds. I don't remember anything being said about a loss of potency, though.
Good luck.