what is normally missing for most people in their training routine to build muscle, and i find this is particularly true for women is:
1. Eating enough calories to build muscle. Most women are so frightened of getting fat they never eat enough.
2. Overtraining. Weight training is new to a lot of women, so they spend hours in the gym, one hour on the x-trainer, 30 min on the treadmill, then 1.5 hours lifting weights. 6 times a week. Way too much and pretty much a formula for failure.
3. Not training with enough intensity. You are not going to build muscle waving 5 lb dumbells around for three sets of 12, unless that weight feels seriously heavy to you (which is possible as a lot of women are also lacking in upper body strength).
Seriously ladies, most of you have gone shoe shopping and carried more weight around all day than you move in the gym.
It isn't pretty and it isn't glamorous when you train balls to the wall, but it is what will give you muscle.
When you start to understand why some of the guys in the gym are making all those horrible grunting sounds when lifting, and you find yourself doing the same thing, you are training at the right intensity.
4. Sleep. Not enough rest to recover.
Yes, steroids are used, but they are called performance-enhancing drugs for a reason.