It takes so long to build muscle because your body fights it. The way that the human body has adapted/evolved is to conserve all possible energy, having said that building muscle takes ALOT of energy.
You will find that some people will generally have better genetics for putting on mass than others, luckily i have fairly good genetics but i know what you are talking about as many friends of mine (unfortunately) dont have genetics.
Theres no key to gaining muscle, all of the people saying it is easy are stupid, just because it is easy for them, does not mean it is easy for others.
the best things to do are have 3 shakes a day one before you wake up another pre-workout and the last post workout, whilst eating 4-5 meas a day with plenty of good carbs and lean protien.
Workout-wise i would only work each muscle group once a week to let it fully recover (and avoid over-training) and make sure you include the 3 best compound exercises all at least once a week (bench,squat,deadlift).
another thing which is good to do is get a spotter and push your reps beyond failure, those last couple of reps will make the gains.
...However this is the gym, everybody has an opinion and thinks that theirs is the best, sample em all and find the best for you.