The key to adding mass to the arms, as well as any other area of the body, is to train to positive failure no matter what rep scheme a person is using. For example, if a person is using a weight that he/she can only do for 12 reps, then the 13th rep should be impossible to complete without help from a spotter. That is positive failure; when you cannot complete the positive part of the rep without assistance. Typically, I like to keep my repetition range from 5-12 but I will occasionally do some high rep sets, 15 or more reps, so that I may stimulate as many muscle fibers as possible. I like to go as heavy as I can without sacrificing too much form. Toward the end of the set, I may employ a controlled cheat if I feel that this will help me squeeze out a few additional mass building reps Another thing to keep in mind is that even if you are doing the greatest workout with the utmost intensity, you will probably make little if any gains if you are not eating/supplementing correctly and getting enough rest/recuperation. Lifting weights is only a stimulus. The body will try to respond by becoming stronger and increasing in muscle size but only if you give it what it needs. Getting bigger and stronger takes place outside of the gym with proper nutrition and rest. What a shame it is to see people training hard in the gym day in and day out for years on end with absolutely no improvements to show for it.