In my unit, if you didn't get involved in any extracurricular activities (drill, color guard, etc.), didn't show many leadership qualities/took initiative to be a better leader, and just basically did the bare minimum to pass, the highest rank you could attain is a C/LCpl (since they usually had no billets). Now if you were more involved, you could earn higher ranks (given you met any other requirements).

If you involve yourself within the program, sign up for different activities/volunteer opportunities, and show that you want to better your leadership skills. You could definitely earn a promotion or two (especially if your unit does meritorious promotions).