I have tried to teach my 2 cats how to use the pet flap in the pet gate leading to the basement. They keep trying to go through the bars of the gate instead of pushing on the translucent swinging flap. I have showed them and showed them, sprayed catnip spray on it, etc., but no go. I have to keep a small bungee cord strung across it, anchored on the bars, to keep it slightly open. Then they have no problem going up and down the basement stairs. This is puzzling, as otherwise they are super-smart cats.
They are cats, and they are messing with you! LOL!
Maybe they don’t like that first down-step on the other side of the flap going down to the basement. If it were my basement, I’d try and tack a landing platform board on the basement side of the opening so it’s flat and then start them out by going through the basement side, first.