I have a small issue with my threaded headset and for the life of me cannot figure out how to solve it. Would be very thankful for any tips! I built up a classic road bike last year and at the time asked my LBS to install Cane Creek 100 Classic headset. I had no issues with it for a while but recently have noticed that it tends to come loose after every little while, making my bike rattle when braking. I have had threaded headsets before and have always managed to tighten them properly. Yet, in this case I cannot seem to lower the upper lock-nut enough to keep the threaded top-cover in place. There appears to be a small washer between them with a hex screw slot. There are also small threads still visible beneath the lock-nut , but it just does not go lower. At the moment the threaded top-cover has so much room that at the uppermost position there is at least a couple of millimeter of play in it. What's the trick here? Should I find a hex wrentch small enough and try to loosen up the washer? Should I try to find another washer to install in-between? Cheers!

what is likely happening is that the top of the locknut is bottoming out on the top of the fork
and not allowing it to press against the threaded top cone or race to prevent loosening

to solve this

either
add a thin spacer between the top nut and the next component down
or
cut a few mm off the top of the fork

no matter how you solve it
make sure to leave enough of the threads of the locknut engaged that you dont risk stripping it out when tightening