Hello, my name is Dr. Farwell. I will try to help answer your question about Misty.
Sorry to hear she had a bad experience with the cat flap (cat door I presume).
It's definitely possible for a kitty to get past a bad experience like this, especially if the it's a different cat door entirely.
Can I ask what exactly happened? Did she get stuck, injured or just frightened by the previous cat flap.
Some suggestions would be to gently place her through it by hand afew times so she knows it
Unless she is one to just go completely berzerk
The other suggestion would be to tie or prop open the flap part of the door while you are there to watch. See if she will get comfortable going in and out with the flap open.
You can coax her in and out with food (canned) if she is food motivated. or kitty treats
There is a product called feliway which comes in diffusers or spray. It's a feline pheramone which has a calming affect on cats. I would suggest using the spray on the cat flap.
Hopefully some of my suggestions help. Please feel free to respond with further questions/specifics.
sorry did not make myself clear,misty(cat) uses dirt tray at flat but just stares at door nervous as stray cats from previous home, used cat flap..good suggestions had it been appriate.thanks