I think 12" from your light is too close considering the size of them, only put them that close after about 4 weeks assuming everything is ok. When did you start feeding? I'd start around the 2-3 week mark. You should get them in bigger pots now, they could well be root bound. Hope this helps, good luck!
Oh and yield for 5 plants depends on strain and growing conditions, indoors i'd hope for around half a pound.