I think your triceps are growing fastest in your body. They're doing great, otherwise the transformation is visible but not as fast as in triceps and arms in general.
How is your weight doing? I think extra calories might help you grow faster. You can ceep yourself lean and grow slowly or build up some belly and grow faster... (or be Zack and grow fast while keeping yourself ripped but I wouldn't count on that since it won't work for most people as well as for him)
Since you're still quite lightweight, it should be quite easy to build some more weight. Just by eating for example nuts all the time. You'll easily get 500 calories a day more just by taking 100-150g peanuts or cashews a day while keeping your nutrition otherwise like it has been so far.
"Yeah buddy!! Lightweight baby! Aint nuttin' but a peanut!"
There's so much wisdom in these words.