Friction somewhere is preventing B from retracting. You can put small dabs of grease where the return spring hooks slide against the brake arms (near where you have A and B labeled in the photograph) to help mitigate friction there. It could also be, or is more likely due to, a poor pivot design up at single pivot bolt and this part needs disassembly and grease/filing/fitting, etc. It may be that this specific brake caliper will never work right due to design, but reducing friction points is a good place to start.