I'm going to post a very simple list. I am not a trainer. I just have been reading up how to increase muscle mass. There are lots of theories about how many calories to consume, when to consume them and what type (carb, prot or fat). I'm not going to touch on that.
I read that if muscle growth isn't occuring as desired (such as you've reached a plateau or saw no initial results), to shock the muscles by changing these variables. To add muscle:
1) Increase weight or resistance
2) Perform more reps
3) Perform more sets
4) Move the resistance slower
5) Rest less between sets and exercises
I know in one post you were worried about your arms getting too big. Most women cannot get too big by lifting. They don't have the testosterone to do it. But some women gain muscle mass more easily in certain parts of their bodies than others - like you do your arms. My calves get huge easily and my legs get muscular easily, so I don't do lower body weights at all (no calfs, quads or hams, except maybe for inner/outer thigh). I do stretch those areas a lot though.
I would suggest lower weights and more reps for your arms. Also balance it by finding shoulder, back, lat, chest exercises to do and increase your lower body strength training. Add the muscle there.