First off, what do you mean by, "when the lock ring is fully tightend"? The adjustable cup has to be snugged down to take up any play before the lock ring is tightend. Are you saying the adjustable cup is going in too far to leave any threads for the lock ring, or are you saying the adjustable cup is not threading in far enough to take up the slack in the bearings?

If the adjustable cup is going in too far, then face the fixed cup side and or check that you are using the right size ball-bearings.

If the adjsutable cup is tightening up before the slack is out of the bearings then chase the threads so it can reach the proper depth for adjustment.