what you grow, and how you grow it effects yield..
some plants prefer 1 main stem, some like to be bushed out.. a few runs with your plant should give you a better idea of how she can grow..
taking off the top of the plant once it reaches the light will promote growth horizontally, you have very limited vertical space, so you want to make one even canopy,
you want to do the majority of your training before entering the flower stage as well.. a PC grow takes precisely planned growing..
any pictures of your box? and it setup?