i have the same problem w/arms.
i was sitting at a 42" chest and 14.75" arms last yr.
it's been my goal to bring my arms up over the past yr and now they are just over the 16" mark.
i dedicated a day to working primarily arms where before i would work them with chest or back.
i try to completly torch them on that day.
drop sets, heavy negatives, supersets, high reps, low reps, everything all in the same day.
and then take a day off any push, pull or arms excercise the next day to let them recover.
also, i did have to get bigger everywhere for my arms to grow - but proportionally they grew the most.
i feel that big muscle moves like squats, deads and some olympic lifts helped.