How do you do step 14? Is it the way OLC intended, mouseclicking the TDC button so the dial panel is removed? Or a keypress assigned to auto-update with SetKeys? Maybe the TDC wasn't propperly set to auto. I stick to OLC's way, no TDC-dials visible means it's on auto. If you fire while scope-locked and the dials are still there, then their gyro is set wrong. Usually the reason why I miss the intended spot or not at all.
What the others said about measuring speed if the scope is not along the uboot's bow or stern is the most probable cause of your error. If you are moving 1 knot and the scope is on 315 or 45 degrees instead of bow or stern while viewing, you cause the scope line to move 0.7 knots sideways. That's definately going to distort a speed measurement of a slow target (6 knots), enough for atleast a displaced hit.
And naturally I promote the use of the 58th degree trick to get the speed value!
[Edit: OLC mentions course-steadyness. Hadn't considered that yet. Thanks]