Blueair sense + motion sensor issue

Hi , I have a blueair sense +air purifier which should operate by waving your hand over the top , I got it apart and removed the board with the sensor on , it all seems fine and operates fine up to a distance of 6 - 7 inches away  but when it goes back behind the glass it does not work , any ideas anyone , here is a photo of the sensor , seem like 2 led's and a sensor , not quite sure how it works .