We have three 6 month old kittens. Two kittens use the catdoor but one kitten 'can't'. We taped the catdoor flap up for a while then introduce them using either their head and/or paws to open the catdoor flap. Two kittens understood it but one kitten doesn't push hard enough. We've tried to entice him with treats and toys but have not had any luck. Does anyone have any suggestions?