Hi auto. What kind of soil/mix are you using? How often are you watering? What kind of pots? Enough drainage in them?
I've seen a lot of great grows in just one gallon pots! Hell, you ought to see the plants they grew in beer cans! I'm thinking that perhaps they are not getting enough water. Just my .02. Let us know your soil.
As far as purple stems......I freaked on my first grow as they all purpled. On my current grow, 2 of my girls have purple stems and they're fine. It could be a deficiency but it could also be a pheno of that particular plant.