Why are dry ice crystals, or 'Snow,' building up in my test chamber?

A build-up of dry ice crystals, also known as snow, occurs only if an optional CO2 system is installed, and this system is operating below -100°F.

Check to see if the temperature controller is out of calibration.  Then properly calibrate the temperature controller.

Some snow is always present in the range of -75°F to -100°F, but it sublimes as rapidly as it forms and does not accumulate sufficiently to be objectionable.