RE: Why can't I pull more weight?
To add strength you may want to join a gym or get some free weights for at home and start a weight training program. Concentrating on upper body work such as arms, shoulders, and back can improve the amount you can draw.
Another exercise that could help would be to simply draw your bow and hold as long as possible...then rest....then repeat...etc. After time you'll not only be able to hold longer but you should be able to up the poundage that you can draw.
Hope this helps!