I was having A LOT of trouble with the 5 lb bow I made for my son, and when I finally got it tillered correctly, I found that it always shot way to the left. I let my son shoot it (he was 4 at the time!), and he put every arrow into an area three feet wide and five feet vertical. The deviation was due to inexperience, but they weren't all to the left. Then I really looked at how I was shooting the bow. It turned out that because my draw is almost twice his, I was holding the bow and string really weird. When I made a concerted effort to shoot it nice and straight, and carefully draw it, the shots started consistently going into a two foot circle, right in the middle.
This case may be different, but that's what I experienced. Good luck, post some pics when you're done!