1.) Not enough protein. You absolutely need to eat what the body requires or you will not build muscle. It's that simple.
2.) Don't overtrain, in this sport less, is truly more. If you feel the urge to do a little bit more, STOP. Don't inhibit your progress because you think you need two hour workouts.
3.) Water, Water and more water or gatorade. You need to keep from dehydrating, so the only thing that should be in your mouth is something to restore your electrolytes.
4.) Form, sloppy form builds sloppy physiques. Always use proper form, even for that last rep or set of the day.
5) Cardio. You absolutely need to do cardio. You can't run out of gas half way through a workout. On your off days get your butt in the gym and run it off for 25-35 minutes.