Nature, God, fate or another instance (depending on what you believe in) is not fair. Some people build muscle mass easily, others struggle for every kilogramme. Some people are born athletes, others are fighting for the smallest progress in strength training, cross-country skiing or swimming. Here are some tips for building mass for skinny ones - often referred to as ectomorphs.
First: "start eating instead of putting blame on your genes"
In most cases ectomorphs like to put blame on their "poor genes" for no effects. This is the simplest psychological defense, "it's not ME to be blamed, it's my genetics!". Instead of looking for excuses look for the real source of problems. Ok, let's assume that you have an extremely poor genetic predisposition to build muscle mass but... tell me: what do you eat? I am often asked similar questions: "what should eat a person weighing less than X kgs?". But when you want to discuss your eating history, there is nothing. The situation resembles that of a joke about winning the lottery when the protagonist laments to God that "he cannot win", and the sky opens and a voice says: "you fool, give me a chance, buy a ticket!"
Recently I asked a 70-kilo man about his diet, and I heard that the most anabolic products include: 2 litres of milk and some sausage. Of course, the man said that he eats a lot of them. If you do not cover your daily needs, what do you count for? A miracle? You won't build stunning results on a sausage and highly processed products. If you do not have tapeworms or other parasites, metabolic disease, hyperthyroidism, cancer, if you are not destroyed by radiation sickness or drugs - then surely beyond a certain calorie intake you will start to grow. If you want to be sure about your health, do a set of basic medical examination. Often, simple urine and blood tests, plus determination the level of basic hormones (testosterone, estrogen, cortisol, thyroid function) can say a lot.
You've added calories and there's still no effect? You keep eating too little. Increase the amount of carbohydrates until you start to gain fat: you should notice a couple of extra centimetres around your waist. Often when one hears from their coach that they eat "too little," they usually abandon his advice and start to look for: "miraculous pills", "special training" and "mass building anabolic secrets".
To sum up: genes are responsible for many aspects of your life, they tell your body how to respond to training and how quickly to regenerate. To a large extent muscle fiber composition and moto-neurons reactivity distinguish a marathon runner from a weightlifter. But... you have an enormous influence on how your body will evolve. Don't hide behind genetics, face dietetics!
Second: "simple methods are the best"
It often happens that beginners look for a sophisticated program designed for advanced bodybuilders assisted with a number of pharmacological agents. Right away forget about trainings scheduled six days a week where each part of your body is trained separately. It's good for really heavy and experienced athletes. This does not mean that a similar program will not work, but according to research it is possible to increase arms musculature without its direct stimulation . Why do you want to spend 45 or 60 minutes just on triceps and biceps only, shoulders or front thigh? At first you will note huge increases even with minimal training volume (volume means amount of exercises and a number of repetitions in a session, eg. 10 sets of 10 reps of squats gives you the volume of 100 repetitions per training unit). The FBW program (full body in one session) can take you as much time as an advanced sportman needs to devote for 2 muscle groups. Similarly I decidedly advice against a program split into three days. If you have to, employ a training split for 4 days (ie. upper body/lower body) or comprehensive plan: full body workout for 3 days.
The worst thing you can do is to spend hours analysing details which are completely irrelevant to your level of advancement. Wondering how to accentuate work of the medial head of the quadriceps but you haven't built elementary strength and mass of your thighs yet? This is not your way. You wonder how to bring out the tip of the biceps but you have less than 40 cm of arm girth? Where is it going to lead you?
To sum up: Choose a simple plan based on a bar, barbell squats, deadlift, rowing, bench press and shoulder press variants, lunges, arm curls. The less machines: the better. The less isolation: the better.