What kind of lights are you using? If growth is stalled and they look healthy then nutrients are not the problem at all, and they're not old enough to be rootbound in containers that are close to a gallon size. I suspect you either don't have enough light, or you have too much heat and they're spending too much energy just sweating. Or something else like that. Any bugs in your grow? Little gnats flying around that you didn't think much of?
I grow 2.5ft tall plants in far less than half gallon contaners with just shultz type crap nutes.
heat- not a problem, stays at a average 80 degrees lights on.
lighting- not a problem, I have 11 CFL's 8 23watt and 3 40watt PLUS 2 T8's
bugs- not a problem, have checked my plants every day, no gnats of any sort, no spiders, nothin.
They have not COMPLETELY stopped growing, they have just slowed down alot.
This is my first grow tho... so idk if its common for them to slow down after 3 weeks or not...
Also a note I should add is, they seemed to slow down when I LST'd them, the side branches are growing nicely, but the top has failed to grow new nods.