I found the same issue with my two kitties. Have you tried getting down there and pretending you're a kitty, pushing the door slightly with your head to show them that it does open despite the resistance? Do it multiple times while they're watching. (Yes, the things we do for our kitties...) If that doesn't work, try opening the door with your hand (or whatever) wide enough for them to get out, and then slowly decreasing how far the door is open. Once they're part of the way out, let the door down very gently so it touches their heads, so they get used to the feeling.
I have to admit that this did not work for my kitties, but maybe it will for yours.
If they need additional enticement to go through the door when you're holding it open, you could try putting a bit of catnip or some treats on the other side.