Nope only time it should be flush is when you first put it in after you have put new pads in. After that it sits proud, try CAREFULLY to push the piston out a short distance, use an air line if you have one, put something inside the caliper so the piston doesn't come right out, old brake pads are ideal for this, and check the state of the piston to see if it's rusty, if not then that's probably not your problem. Have you checked the front wheel bearings?