1) You simply aren't eating enough food
It's not just protein that you need, you in fact need a lot of calories coming from protein, carbs and fats. If you take the suggested average calorie intake for a male - 2500, to lose weight, you need to consume less calories, but to gain weight you need to eat more. You need to add at least 500 or more calories a day to start gaining size. Your hard work in the gym wont matter if you're not getting enough nutrients.
2) You're not training hard enough
I think we have all seen them - people in the gym 'going through the motions'. Those are the people that won't get any real results. If you want to be big, strong and lean, you need to push your body to its limits! I'm pretty dam sure Dwayne Johnson doesn't just 'go through the motions'!
3) You're doing too much cardio
Performing too much cardio will make mass building virtually impossible, especially if you're already not eating enough. Burning more and more calories is pretty pointless when you want to get bigger - to get bigger, you need more calories, not to burn more.
4) You're not training right
If your goal is to get bigger, then you need to train for that. It seems that most people (trainers and fitness enthusiasts) would agree that 3-4 sets of 8-12 reps is best for hypertrophy. You want to concentrate on correct form and hitting muscles from different angles.
5) You're not getting enough rest
When you workout, that is NOT the time when your muscles grow, its actually the time they get broken down. Nutrition and rest is what causes them to get bigger and stronger. A lot of people don't get enough rest in between workouts, which hinders muscle growth. Make sure you get enough rest, sleep and correct nutrition.
6) You're not getting enough protein
This one is pretty obvious, but it seems as though many people don't get enough protein. Trainers and athletes that I trust recommend around 1.3-1.5g of protein per pound of body weight. You don't want to stick to just one form of protein (i.e. chicken). Eating a variety of protein foods can be highly beneficial to muscle building. (different foods contain different amino acid profiles)
