some ppl claim to be hardgainers because they don't realize how hard it is to actually put on muscle
but some people are genitically doomed when it comes to putting on muscle. Those with tiny frames, bone structure etc are never going to be huge and do find it extremely diificult to put on muscle. These people are the only 'true' hardgainers.
Heavy Bench Press, Heavy Deadlifts and Squats are the 3 Big Muscle Building Exercises. Ensure that you perform them without fail. These Multi-joint exercises involves the largest Muscle Groups forcing your body to grow
Perform 8-12 sets per body part and keep rep range between 4-7.
Exercise each body part not more than once a week and each training session should last for not more than 60 minutes.