If you have done the standard tonearm setup stuff - zero balance, set balance, set antiskate - maybe increase the inertia of the tonearm by adding weight to both ends of the tonearm to increase its mass while retaining the ability to get the tracking force correct. That could help if the arm is too easily knocked vertically by something in the groove. If this is the unipivot arm any modification might get tricky since the arm can roll on the pivot as well as pitch and yaw. In that case more weight vectoring straight down on the unipivot might help? This would require pretty precise positioning.
Of course you could just try increasing the tracking weight.
