besides diet, you need to look at workouts/recovery time/volume. you can work your ass out all you want, but if you plan sucks you won't see growth either.
definitely diet is the most important part, and after that you still need to be doing the right amounts etc.
lotsa ppl go to the gym and work their ass off, but working your ass off does nothing if you don't know what your doing. i.e. curling 3 times a week, etc.
Post your workout routine, if your not sure about that or look at ******* or something. not making any gains after 3 months of doing the same exercises shows something is clearly wrong, even if you diet sucks something is wrong