Arms take a beating if you don't watch it. I've learned to cool it on 2x a week workouts focusing strictly on arms. Over worked arms ruin my chest workout every single time so in my old age of 33 I've had to focus on proper rest and lots of stretching....seems to have worked because I'm getting more and more work done with great satisfaction on bi's and tri's.
Rest
stretch

Yeah took me forever too I found doing supersets from bi's to tris, and always ending in drop sets at the end of the workout, also i found weighted dips with different spaced grips smashed my tri's to a whole new level. I now start my arm day out with muscle ups now for a compound movement then move to isolations and end in dips and pull ups to really make sure they are dead at the end. But i'd dare say it may have something to do with how you structure your workouts all round and how often you hit arms etc. I do arms once a week after a week of compounds No1said they get plenty from back and chest w/o s.

My arms too forever to get descent. For me i had to blast em to hell. All angles for Max pumps. Stiil trying to get that cool peak but....still trying. Preachers, hammers, reverse grips, close curls, thinkers. Tri extensions neutral grip, palms up, palms diwn, narrow v bar press, ropes, donkeys.
Arms smaller since brokke elbow....5-4 17 3/4......was a tithair over 18

I tend to prefer a HIT approach with my workout. Pre-exhaust with a heavy warmup, followed by mainly isolation exercises. I do a lot of negatives. I don't do negatives all the time at the end of each workout, because it leaves me feeling to fried all time. But every once in a while to shock my system. With arms I mix it up between dumb bells and machines. If I am working biceps I like to work down the rack. By the end I can barely lift the tiny ass weights, so new people walking in look at me weird lol