you need to start slow..... how long have you had her? I have a rat, have had for 10 months now, she still is skittish. Its her temperment. I have spent alot of time, with my hand in the cage, so she gets used to the scent. Also, talk, talk talk talk talk in a nice soothing voice. And the better the treat, trust me, they are all for that. Meat is a big favorite for mine... bolonie, ham.... not to much.
She may also be so loney. What are the chances of getting her a friend. A female friend. We dont want youngins
it just takes time. And there is no definenate time frame I can give you.... its up to the individual rats. So, keep at it. She will come around to you. It just takes time.