It could be the piston "cocked" or the brake hose is collapsed internally or that the caliper is seized. Crack open the bleeder and see if the piston retracts. If it does, you may need a new brake hose. If it doesn't, try loosening up on the clamp and have someone step on the brake pedal and watch to see if the piston pushes out (make sure your brake drums are installed in the rear and the opposite caliper is not left hanging without a clamp on it before you do this). If the piston extracts, recenter the C-clamp and try again. If not, your caliper is likely siezed and should be replaced.