I have always had problems with making size gains. I eventually concluded it was probably just genetics (easy excuse, right). After doing a lot of lurking on the AM forums I began to think twice about this. Somebody mentioned using fitday.com to keep track of their calories so I thought I would do the same.
I was pretty shocked with what I found out: I had probably been eating about 1700-2000 calories a day. No wonder I couldn't gain any mass! I have quickly seen a difference in body compostition in only a few weeks just by bumping up my caloric goal to about 2500-2750 on a 40/40/20 macro ratio. My weight has gone up slightly, but I actually have more striation in my muscles and my pants are more loose around my waist.
Moral of the story? I was considering using supplementation (even possibly AAS), but with some good research I found that I have a lot of room to improve naturally (I'm 5'8 and floating around 159...lots of room to improve there).