...though if you only have the one bulb and ballast, and they're already under 600w of HPS (should be around 90,000 lumen), I'd keep 'em there but maybe place a coupla/four CFLs (warm white/2700k, big as ye kin git) around the edges, to 'round out' the spectrum that the HPS might be missing. And keep POUNDING 'em with the HPS.

And are they still in 18/6 lighting, or have you moved them to 12/12? Also, how far is the light from the top of your 'canopy' of green-ness?

You might have a 'lowryder' or heavy indica strain there that doesn't /get/ tall. If they've 'stopped', tho, it still might be the 'too-small pots' issue... And Rusty's right about keeping the humidity low... I heard something about keeping it to the low end of the spectrum, because that tickles the plant to exude more oils to keep the dryness at bay.

Bushes are kuul.

I like bush.