1. Not enough lifting: you have to lift weights like a muthafukka Heavy weights, lots of reps, lots of sets, lots of angles. In the gym 6 or 7 days a week. Muscle groups getting hit 2 or 3 times a week. Consistently adding weight to the bar. Benching and squatting week in and week out. Period.
2. Inadequate protein consumption. Eat meat, eggs, milk, protein shakes all the time. Every 2 to three hours more protein.
I can put you on a lifting program and I can write you a stack of creatine, tribulus, d aspartic acid and whatever else. But if you're not taking the above 2 steps, forget about it.