If I had to pick out the number one reason most people fail to achieve good results in the gym, it would be
Over-training, and Exercising too much. More is not better when trying to build muscle mass…

A few more that I might add to your list are overtraining and focusing too much on isolation exercises instead of proven mass builders like the bench press, squat, etc. As many of us know, but also many of us forget, its these multi-jointed exercises that are really going to help pack on the mass, not the preacher curls!
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