Food is the most anablolic substance known to man. You need to eat LOTS of food to gain weight! Bottom line is, if you don't eat 6 times per day (meals as well as supplemental
protein drinks), you don't grow. When you build muscle (which it sounds as if you have already done) you increase your metablolism. So you need more food than you normally would to simply maintain your current weight. What you should do is keep a daily log of everything you eat for 3 days. Then figure out the calories, protein, carbs and fat that was consumed during that time period. Add 500 to 1000 calories more per day of good, clean, high
protein foods. If you do this you should start to see some weight gains within a few weeks. Good luck!