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Switching things up can help sometimes. Even changing equipment. I prefer the 12-15 rep range to failure for many of my sets on arm work. I also like to use dropsets, even ones where Ive hit 85 straight reps. I do a minimum of 12 sets for triceps and 12 for biceps. I can relate to your problem. My arms were stuck in the 17's for four years! I only gained 1/4 inch per year during that period. My training was very intense as always but the gains were very slow. Im not sure why it took so long,but it eventually came to pass and I hit 18". Now In the last 6 months Ive gone from 17 3/4" to 18 3/4", nearly 19". I ended up taking a long lay off when I hurt my back a year ago, and I only trained my arms for a month, about 2X a week. Couldnt train anything else. I gained a quarter inch just in that 1-2 month period. Then my gym closed down and I started training at another gym, that combined with low doses of M1T and Ive gained an inch in 6 months. I train arms atleast once a week, and sometimes biceps get hit twice if I do them on my back day. Adding in hammer curls also may have helped. I do alot of pressing which may help with my triceps also, but they are still my weak point. The best advice I can give is to be persistent, maybe try some changes in the routine, and eat even more if possible. I try to keep my protein at a consistent 200 grams per day, but dont always make that either. Fucked up intestines dont help. Not sure of my bodyfat, but Im around 200. Heres some pics with my arms at 18 1/2", theyre slightly bigger now. Dont get too discouraged, try to look to the end of the year to chart all your progress. A 1/4 inch a 1/2 inch, it all adds up. Be sure to keep a chart once a year with some pics, helps alot.:
