1 – You’re not eating enough.
2 – You’re not stimulating the muscle correctly.
3 – You’re not eating enough.
4 – You’re not resting/recovering properly
5 – You’re not eating enough.
Bonus – You’re still not eating enough.
Bulking, as it’s more commonly known is actually no easy task, adding lean muscle takes time and a willingness to sacrifice those abs of steel for a period of time to allow the correct caloric surplus for your individual needs. Remember being in a surplus is key to building muscle, however you may indeed add some excess ‘fluff’ as well. A tough pill to swallow, but it’s true.
What kind of calorie surplus will you need?
About 500 calories as a guide, some need more, some less. It will take some learning the correct application to get right for you as an individual, but as a general guide +500 each day is a good place to start.
That’s +500 on top of your TDEE (total daily energy expenditure), by the way.
If you have been trying to gain weight for years and been unsuccessful then it’s suggested that you hire a coach to help you. Often the hard thing, from what people say, is the following “I don’t know what to eat”… Food. Food is what you should eat, sadly there is no magic meal plan or supplement that will grant you instant muscle, it doesn’t work that way. You need to get your daily calories right and then the optimal macronutrients as well, here is a nifty link (http://www.iifym.com) to help get you started, although hiring someone would be the best possible option to help with planning, adherence and motivation.
Now go and start building the muscle you deserve.