I own one pet bunny and three pet cats. All in which are being taught how to use a doggy door that I installed about a month ago and both the bunny and two of the other cats learned how to use upon the first day. However, I've been noticing that one of my cats in particular seems to still not understand how to use the doggy door. I've been trying everything, seeing articles online that it's best to leave it wide open upon the first days, teaching the cat that they can go in and out, attempting to even reward with treats, so and so, yet he still has not grasped the concept of the door. Usually I see people online saying that sometimes it's easier to simply just remove the plastic fold / door. But we are trying to avoid this only because we dont want certain temperatures from either or room to escape. He has watched his two other cat brothers go in and out constantly alongside with the bunny, but still thinks it's something that only the other cats and the bunny can open and not him. Keep in mind, this cat is the smartest one out of all the other animals. He knows how to open and close cabinets, even knows how to push a creaking door wide open. And he is the only cat who actually knows how to do tricks, alongside commands like sit and roll over, yet, the doggy door seems to be a big challenge for him. Does anyone have any good advice for any sort of similar past experience? 

My cat still hasn't been able to figure out how to use a doggy door