Even if you are already doing plenty of back training, to really target and grow the lower traps, add in some direct lower trap training. Based on how much volume you're doing, your recovery capabilities, and schedule, you can train your lower traps anywhere from 2-6 times per week. For most people, a frequency of 2-3 times per week is the sweet spot. You can start with 2 times per week and as your volume increases, you may add another day if it fits your schedule.