yeld issues

basicly heres the deal my plants don't produce like I think they should. vegitation is ok and they seem to flower ok but on a four foot tall plant with five to ten tops I am only getting one to two ounces when dry. I have three different strains, and all I have seen produce seven ounces or more on the same size plant. I have six plants under one 600w hps and one 600w mh on the flowering side. I use fox farms line of products incuding the open seseme, beasty blooms, and cha ching. the buds are dense and are very good but I would like to get more per plant any suggestions?

One good trick that I think should help is, tie the plants over. Take the top of the plants and tie it over at like a 80 degree angle, with some string or whatever. This should increase light to lower branches, I have also heard something about a hormonal shift or something when you do this; but I don't know anything about that.