I've got a stock Ritchey threadless headset on my commuter. The bike is new this year, with ~300 miles on it by now. After I bought it from the LBS, I noticed the headset was loose, so I adjusted it and it has been fine since then. This week, I noticed it was loose again (braking a bit jumpy and the characteristic "knock" when front brake is pulled and bike is shifted forward and back). I tried adjusting again but I absolutely cannot seem to get it tight--including when I way over-tighten it. What I mean is, no matter how tightly I cinch it down, I can't get rid of these symptoms. I've done this adjustment a good deal on other bikes, so I'm operating under the assumption that my technical skills are also not the problem.

I've checked and the star nut isn't stripped, front hub adjustment is good.

What's going on here? Hardware failure? Low quality components? User error? Inaccurate understanding of how to check for proper headset tension?

Thanks all