Diagnosis
The new filter is not being detected. This is usually an installation error, not a faulty filter.
How to Fix It
Confirm the filter is the exact model for Blue Pure 211/211+: the part number should include 'SmokeStop' or 'PARTIKEL'. Remove it. Check for and remove any clear plastic shipping film on the filter—some batches have it. Ensure the black plastic frame is perfectly intact with no cracks. Reinstall: align the filter with the guides inside the unit, press it firmly and evenly against the back wall until it sits flush. Close the cover until you hear a definitive click on all sides.
If That Didn't Work
Try your old filter. If the old filter works (white light), the new filter may have a defective RFID tag or be for a different Blueair model. Contact your retailer.
Heads Up
The filter detection uses an RFID tag in the filter frame. Placing metal objects or strong electronic devices very close to the unit's back can potentially interfere with the sensor.