Some of the best advice I have heard for people who complain about not being able to gain mass or weight is from an MMA forums. Most people who complain of not being able to gain mass are eating less than what is needed to sustain a dead rat, at the same time they are either lifting too much (frenquency wise) with too little rest, or too little (low weight, high reps, or low weight low reps). The advice from the MMA forums is like this.
Get a watch with an alarm, set said alarm to go off in two hours. When it sounds, eat good growing food (good fat, good protein, good carbs look elsewhere for a definition of "good") until you feel sick (literally feeling sick for two reasons, one you are used to probably 1200-1800 calories in a day, and two you stomach will literally need to start training for actually eating like you are lifting). Do this every two hours until for a few weeks until you have trained yourself to actually eat.
Hell this advice in polar opposite (relatively small meals with great frequency) probably works for cutting as well as bulking. Oh yeah, if someone tells you are a hardgainer, run away rather than telling yourself you are a cripple for your entire life and that you can't gain muscle work around it.