Now, this goes for anyone who wants to develop new muscle tissue.
Wether that be bigger arms, fuller chest, wider back, toned legs or a bigger, firmer bum. So this goes for both guys/girls/anyone wanting new muscle tissue.
It can be a slow process and it takes some serious patience, especially when you've been training hard for a few years already.
It could be that....
1. You don't eat enough. Starting simple here. If you don't fuel your body with a surplus of calories you're not going to give your body chance to grow. You need food to grow. Track your intake, increase it when you need and monitor the effects.
2. You don't progress your lifts. New muscle requires an increase in training volume (reps x sets x weight). If you don't provide overload to your muscles, there is no reason for them to develop.
3. You aren't training hard enough. Similar to above, you have to create enough stimulus to your muscles to warrant a change. Going to the gym and actually pushing/progressing yourself are two completely different things. One will produce the outcome you're after. A lot of us think we train hard, but there's too much left in the tank.
4. You don't know if you're growing or not. A large percentage of people who say they aren't growing have no idea if they actually are or not. Are you tracking your progression? Do you take progress pictures constantly? Do you weigh in weekly? Do you measure circumferences? If not, you can't complain that you aren't growing because you will have no idea if you are or not. Accountability is key.
5. You're starting body fat is too high. This can cause problems with both resistance to food, making it more difficult for your body to partition nutrients and use them efficiently, and also knowing where you're body weight is actually going. Visually it's very difficult to tell if you're developing new muscle or if you're gaining more fat making it a quest to bulk for the sake of bulking, resulting in little to no muscle at all.
6. You're hanging on to being lean. Doing cardio to keep body fat low whilst trying to grow, to make it a 'lean bulk' is senseless. This one makes no sense at all and hinders your progress massively. Growing requires a calorie surplus. Losing body fat requires a calorie deficit. Pick one and zone in to that goal. Accept that a little increase in body fat is required, inevitable and helpful.