That was my thought: the star-nut may have slipped. I have had five CK headsets on my various bikes (including on two tandems) for many years, and the only problem I've ever had is the star-nut slipping. IMO, the star-nut is the weak link in an otherwise well designed Aheadset/threadless system.

You can buy inexpensive star-nut setters from Nashbar, Performance, etc. Nice to have in the toolbox, and automatically sets the star-nut at the correct depth. (Note that they don't work too well on Santana 1.25" steerers, as they are made for 1.125 steerers.) If your star-nut is slipping, best to just get and install a brand-new one. Once they start slipping it's hard to get them to stay put.