You probably need to change your workout. The trick to building good muscle is to constantly confuse your muscles. One of the reasons why you got such good results when you first started working out was because it was new to your body. Create a new exercise program every 3 months.
1) stay away from weight gainers.
2) keep taking a high quality protein powder
3) change your workouts every 3 months. Do new exercises for each muscle group, and perhaps do them in a different order. Have fun jock.