Building muscle requires many variables to be activated, and these get harder with advanced lifters.
1) Your training should be intense enough to induce enough stress on your muscle for it to rebuild itself bigger.
2) Your diet must be sufficient to support your growth. Remember when you've already added muscle mass to your frame, you current maintenance caloric needs are increased, so you need to push beyond them.
3) Genetics, believe it or not plays a crucial role in determing the amount of lean mass that your body can grow (naturallt)
4)Rest and recovery, forgotten most of the time by bodybuilders, are pillars to your overall growth, without them it is difficult to see results or even push your maximum at the gym.