Diagnosis
The charcoal pre-filter is likely misaligned or folded. It is a fabric sleeve that must be stretched smoothly over the True HEPA filter. Any bunching or misplacement will prevent the grill's locking tabs from engaging.
How to Fix It
Remove the grill. Remove both the True HEPA and the charcoal pre-filter. Place the True HEPA filter on a flat surface. Stretch the charcoal pre-filter sleeve over it completely, ensuring the sewn seam is straight and the material is taut with no wrinkles. Re-insert the filter stack into the unit, textured side of the HEPA filter facing inward. The grill should now close flush with an audible click.
If That Didn't Work
Inspect the plastic locking tabs on the grill and the corresponding slots on the purifier body. A tab may be broken. If a tab is broken, the grill will not secure properly. A replacement grill (part specific to the 5000 series) is needed.
Heads Up
Never operate the purifier with the grill unlatched or a gap present. It bypasses the filtration path, allowing unfiltered air and debris into the fan, and the UV-C safety interlock will keep the UV light off.