The principle is to prevent other insects, especially pests, from entering the bumblebee's nest. And how do the bumblebees learn to open the protective flap? Bumblebees in the wild can even enter long flowers - they can lift them with their bodies.
Close the flap slowly over the course of a few weeks and get the queen and workers used to the movable flap. The last step - closing the flap - can be made easier with two small drawing pins pinned to the sides of the opening. There will only be a small gap with a passage in the middle. Once you have learned how to remove the drawing pins.
The flap needs to be checked during the year to rid it of dirt, it may be stuck, partially waxed hole, pollen, etc.