Just no curls. Monday - Back/Bi, Wed - Chest-Tri, Fri - Shldr and Leg

I've been working out for about 3 months now. I've been surprised by the quick progress made in increase in strength. All of a sudden however, I have hit this "wall" with curls and am no longer making progress from week to week. I curl once per week and do 3 sets of standing curls and 3 se...