My cats were a year old before we installed a cat flap, so had to be shown how to use it,
One of us went outside and one stayed indoors with the cat. we pushed them thro the flap then the person on the outside pushed cat thro' flap, back indoors, and just kept doing this a few times.
Apparently it helps puts their smell on it, so the recognise it
Just like kids they need to be shown a few times until they get the hang of it.
Good luck!