1. Genetics. I hate the word, but some people ARE built differently. For instance my friend has a great back but lacking chest. I am the opposite.
2. Cut your workout in half and work twice as hard on whats left. I saw "2 sets of this" and "3 sets of that". Try to focus on say, flat bench, and work on increasing your numbers every week.
3. Keep your body guessing. Make sure to change it up every 6-8 weeks, to induce new growth.
4. Try a day where you do 10 sets of 10, to shock the muscle.