You're not doing enough volume.
Everyone's different, but from what I've read your volume should look like this:
Larger Muscle Groups (Chest, Back, Quads & Hamstrings): You should be doing 60-120 reps per muscle per week.
Smaller Muscle Groups (Biceps, Triceps, Shoulders, Calves, Abs): You should be doing 30-60 reps per muscle per week.
I've seen workout routines where you do this much volume twice a week (So you're hitting your chest on Monday for 60-120 reps and then Friday you hit your Chest again for 60-120 reps). Where you're doing each muscle twice a week after a 48-72 recovery gap (which is supposed to be all the time you need for muscle growth and recovery).
Here's a link:
The Optimal Volume Per Muscle Group, Body Part, Workout & Week