You may never pet her, and this is okay! I have 4 tiels and only 1 allows petting, on his terms! It's so important to remember that they are nothing like dogs or cats that love cuddling and snuggling. Most birds do not allow petting, and if you try, you'll get bit. It is just the way they are -- they tned to like their space respected and only want to come to you on their own terms. It's important to understand that they may never want a head scratch or will allow you to pet them and it is something you have to live with -- as long as they are happy and healthy they will not need to be cuddled or pet. If your tiel is not hand fed (raised by humans from a young age) it may be very difficult to bond to a point with your new bird that it will allow head scratches, but if you are patient and don't rush your bird it hopefully will come around to letting you handle it to the point where it will allow a head scratch.