"An only rat is a lonely rat."
Seems females suffer the most from this. They really do need to be around their own kind. I assume you have a decent sized habitat (cage)?
Plus you don't know what kind of shenanigans your rat may have gone through from the feeder-breeder and the store. It may take some time to win her over but try giving her treats once in a while.
This is how a friend won over a particularly timid rat that others had but complained of being bitten etc. She became the 3rd parent for this particular rat. Speak softly in a friendly manner all the time you are around the cage and interacting with your rat. Try putting a small dab of peanut butter on a spoon & leave it in her cage. Do it once a day at the same approximate time because they will clue in on this. If you see her responding positively over time doing this, (will anticipate it) then the next step is to get her used to your touch.
Put a small dab of pb on your finger tip and hold it steady to her so she can approach and start licking it off. Friend did this for weeks before her rat would trust her enough to step up into her open palm. Unlike her other rats this one she never could just pick up or handle, she had to invite her rat onto her palm, from that point she could carry, tickle, and play with it.
Oh, and it turns out her timid rat absolutely LOVES unsalted cashews! It took time trying various treats to discover this. I still remember her showing me this, in the kitchen she would crinkle a plastic bag with regular peanuts and her timid rat wouldn't react. But when she popped the plastic lid off the can with the cashews.. instant response! heheh.. her rat would climb up to the top level of her cage waiting to receive. :)
Hope this might work for you. Check youtube, there are some other alternative ideas that may work better for you.