rear disc brakes? If so, you need a special tool to retract the piston back into the caliper. You usually have to screw the piston in, while applying some force to it as well. This will depend on caliper design.
you have o turn them either clockwise or counter clockwise at the same time as you are compressing them there is two indentions on the piston. i used a pair of needle nose pliers to do it not a special tool.