It could be time, It could be that if you want to make more growth you need to change something.
BB Curls 4x6
Hammer Curls 3x10
Lying Hi Cable Curls to Forehead 3x15ish
Overhead DB extensions 3x10
I never really did anything to pump my triceps because they get much more additional work from other gym movements. Also if you aren't doing chin-ups on your back day, start.
I will say however that this whole thing does not really fit nicely into 5/3/1 but I have always worked arms on a separate day than the four main days so that they wouldn't interfere with one another.