could it be that what that workout has outlined may just be a little too much volume? If I looked at the right one it says in weeks 3-5 you work arms three days a week, and you're doing about 12 sets for biceps and triceps each day! That's a lot. I don't think you have to cut out arms entirely but you may want try it. Of course your arms can still get bigger as long as your doing your chest, shoulder, and back exercises the right way. I would experiment and take the month off from biceps and triceps specific training and the next month add 3 sets of one either dumbbell or barbell curls, and go from there. See how your body responds.