Here's a test. After you've "locked" the adjustable-cup and locknut together, try moving the adjustable-cup underneath by itself with the channel-locks. Hold the handlebars with your other hand. If the adjustable-cup can still be moved, the two aren't locked together. One cause of this is that the tab in the slotted washer hangs up against the sides of the slot and digs in. This prevents it from moving vertically and the actual tension in the locknut is pushing against this washer and not the adjustable cup. To equalize the tension, you want to unscrew the adjustable cup upwards as you tighten the locknut downwards. This works equally well for slotted or unslotted headsets.