Insulators must be very good. Acrylic always works well. PVC tubes are reasonable in dry air. Nylon slowly absorbs humidity and can become quite conductive after some time. The classical material, varnished glass, can also be used, but plastic materials are better.
Bare glass is usually too hygroscopic, and works well only in very dry air.
The same applies to distances from charged areas in disks to the machine structure. Never use wood, cloth, or similar materials for insulation, as they are too conductive. The insulators must be absolutely clean. Clean the disks, and other insulators with alcohol or similar product, removing any dirt, grease, etc. from the surfaces. Some insulation problems are easy to spot in the dark, due to sparking over the insufficient spaces, or corona formation.