Disconnect the noodle and check the arms for free movement, correct if needed. Lube the pivots and also the return spring where it engages the notch in the rear of the brake arms. There are usually spring tension adjustment screws on the brake arms; if there are not or they do not work you can bend the springs a little to give more return tension to the weak side. https://www.sheldonbrown.com/canti-direct.html