PA-28-140 Cherokee trim tab adjustment

[uncle8]

I am seeking an explanation of the operation of the Cherokee trim tab after alteration to the length of the rod connecting the trim barrel to the trim tab.
The Cherokee owner complained that he was using all of the nose up trim when landing with forward c of g and full flaps and asked if an adjustment could be made so that there was more nose up trim available. The mechanic made some checks of the rigging and then adjusted the rod connecting the trim barrel to the trim tab so that the rod was shorter which made the trim tab move down slightly.
A test flight proved that the situation was now worse, all of the nose up trim was used earlier in the approach and there was more back pressure required on final. The mechanic couldn't see how that would be possible so he shortened the rod further, moving the tab further down. The test flight showed that the situation was worse again so the owner asked that the rod be returned to the original position, i.e lengthened so that the trim tab moved up. Test flight showed that it was operating as it did originally, still ran out of trim but much better than after the two remedial adjustments.
It seemed, to all involved, that moving the tab down should reduce the requirement for nose up trim but the test flight showed the opposite. The aircraft is fitted with A.M.R&D. wingtips and stabilator tips and vortex generators on the wings.