I have a 1770NT with 1.6bu boxes. Pro shaft drive. Row command clutches. Row 11 is planting 2/3 of what it should. Row has good vac supply. I have swapped meters, and seed sensors with other rows. Problem stayed with row 11. Changed drive shaft and front hex shaft gear case. Still have the same problem. With the box off the planter the drive turns smoothly. 20/20 shows low population, but spacing is 95% which is less than the others but not terrible. 10% skips 3% doubles. Is it possible that the row command clutch is slipping under load? I can't find any used units to try. Anyone have one they would sell? Is there a way to lock them solid to get by unit I find a replacement?