I thought that all commercial UV shutter self-check flame detectors were NEMA 4, so that the shutter mechanism is enclosed and isolated from the external environment.
The seal around a quartz window should seal the shutter mechanics from the gases and dirt in the sight tube. Is the quartz window sealed and in good shape?
If your assembly is not tightly sealed and gasketed, then purge some instrument air into the enclosure to keep a very slight positive pressure inside, with respect to the outside.
The shutter requires a high voltage to run. You can look up whatever that value is and check to see if the controller is supplying sufficient voltage.