You're getting slow growth, given your specs. I'm suspecting that your pots are too big and your getting root growth rather than top growth. Since you have slow growth going on, and you have an HPS light, I say try to have some more patience and let the plant grow, at least another month. Patience will be rewarded especially since you say you want to go for a bigger yield. Also I'm betting you have no secondary growth yet. If you veg longer you may be able to get some decent branching, and better yield (and do some research on LST), and clone material.

Also think about your next grow...if clones are in your future, you definitely want more growth so you can later have a good selection of clone material. Your next crop then will be alot easier...think clones. Seed growing is a pain in the ass and slow. BTW temps ok? You can get slow growth if your temps are too high (like 85-90F +).

Dboy