I took of the arm and hit hard with hammer but it doesn't do anything. Finally I was able to bent back the brake a bit by doing it while mounting on the bike. The bent may occure partly in the brake bolt itself. Now the angle is about 2mm different between 2 ends of the pad. I thought of the shim but then decide to let the wearing correct the angle. Switch front to back is definitively good advice, i will do that.
BTW, I put on the new red Kool Stop pad and the brake is smoother but squeal terribly. The old hard dry campy pad was quiet. What shoud I do?