I have trouble closing the rear door properly

First, check that the two small red tabs located on the side of the HPP filter are in place to block the filter. To close the flap, use both hands: with your left hand, hold the front corner of the device. With your right hand, press just below the "push to open" inscription. Repeat the same gesture to open the flap. It is deliberately difficult to open and close because it must be well sealed to guarantee the best possible air purification.