The abrasive cutting particles must always be softer than the intended workpiece to be cut.

The metal being cut is likely softer than the matrix used to bind the abrasive particles thus the matrix is not wearing down significantly to expose fresh abrasive compound.

Pure zinc is an ultra hard metal and instruments designed to cut steel will find it challenging to cut it.

Zinc is ultra resistant to all abrasive metal removal processes.

The abrasive cutting wheel/disc in the lab is of suspect quality.