What Causes a Lens to Fail:
Dirty Lens - If a lens has a dust or film buildup, instead of passing the laser energy it begins to absorb it and rapidly fails. Keep the lens clean and even check the lens if you have not run the machine in a while
Inadequate Air Flow - You never want to have 0 air flow or pressure on a focus lens. This can allow dust and smoke film to build up
Moisture - Make sure you have adequate air\water separation and avoid operating your air assist supply in an area that is colder than the machine operating area
Mechanical Shock - Do not drop, hammer, or throw the lens. It will shatter into pieces