Yeah it sounds like the lip on the locknut is bottoming out onto the edge of the steerer, it feels like its tight but isnt actually locking the threaded cup in the correct position of adjustment. I had this on my old Diamondback from new. A deeper washer/spacer sorted it.
Obviously it could be any number of issues but without pics or an in person diagnosis it's hard to pinpoint and fix over the internet.
Riding: Cannondale R800, Thorn Enduro-foco
Hoarding: Orange Prestige, Ozone Oxygen, 92 Klein Rascal
Totally biased, the best bikes in the world are: 90' Raleigh Mirage 91' GT Talera 92 Diamondback Apex 95' Mongoose iboc zeroG 97' Voodoo Erzulie