I'd try switching bulbs first. There's no way that a 400w MH should have a problem with a 2' x 3' area. If your bulb is really old, it may be running low efficiency and not putting out enough lumens. A decent 400w MH should be putting over 35,000 lumens. Since you've got six square feet of floor space, that's well over the 5000 lumens/sq. ft. standard.

Are you using an MH for flowering? I've never done that myself, but I've heard others say you get more stretch and less dense buds. If you've got a switchable ballast, you might want to try a CMH (Cermaic Metal Halide) bulb. They seem like the best "one bulb fits all" answer.