Perhaps some contamination in the clearcoat? I know you said 'polish' but did you try a rubbing compound? I tried the clay bar routine on my car, was pretty unimpressed as a whole. I'll just buff her with cleaner wax if she gets grimy. For nasty stuff, I use a heavy cut cleaner by Meguiars. For everyday grime, it's their cleaner wax and elbow grease.
Since it's an '03 Bob, I would certainly take it to the dealership and have it checked out. See if they can clear it up and if not have the bumper refinished while still on warranty. You may have to push a bit, bit if there is a problem, have it taken care of now.
If you figure it out, let us lnow!
Kate