If the cartridge isn't set to track heavily enough, this could explain everything that's been described.
I would consider also using multiple weights supplied with the Triplanar rather than the one larger weight, so as to move some of the mass further back from the pivot. This would have the effect of increasing the effective mass of the arm, allowing it to work better with a low compliance cartridge.
I would consider also using multiple weights supplied with the Triplanar rather than the one larger weight, so as to move some of the mass further back from the pivot. This would have the effect of increasing the effective mass of the arm, allowing it to work better with a low compliance cartridge.