Here's some suggestions:
In terms of supplements, try some weight gainers if you want mass/bulk up. Creatine perhaps? As a general question, are you eating 6 solid meals a day? Do you consume high volumes of salt or sweetened foods?
As for what to do in the gym, try isolating what you do while ur in there. Say you are on a 4 day per week workout regiment, try doing this:
Chest Shoulders Tri, Back Bi, Legs, Abs. Mixup your workout so that you aren't just doing the same excercises for months on end. Per set, do no more than 6-8 reps.
Hope this helps
