Rabbits simply do not always "breed like rabbits".
Once you have established that you have not been visited by the Sex Change Fairy and that your doe is indeed a doe, it is time to look at things from her point of view.
1. How old is she? If she is young, a first-timer, she may simply be scared or too immature to be interested. House her next to the buck for a few weeks and let them get acquainted through the wire. She will likely relax and start to show signs of interest.
2. If she is over a year old, how is her condition? Is she fat? Is she bouncy and happy or does she just lay around and eat and sleep? Fat rabbits, male or female, are usually not interested in breeding and often unsuccessful even if they finally do the deed.
3. What are you feeding her? If she is on mainly pellets the problem could be nutritional. Feeding Vitamin A rich foods like fresh parsley or dandelion greens and Vitamin E rich foods like BOSS can make a difference. The apple cider vinegar in the water often helps too. Pellets are "complete" when they leave the factory, but may lose nutrients in storage. And those essential ones, A and E, are the most vulnerable to damage.
Before you pop her in with the buck next time, flip her over and take a look at her vulva. The colour of her genitals will give you some indication of her probable receptivity. White to pale pink... not today. Deep pink to reddish... maybe. Dark red to purple... hot to trot.
I would personally not recommend forced breeding or holding the tail up with string. There should be no need for these things. Sometimes a doe that is uncooperative in the buck's cage will happily breed on neutral territory, especially in a larger exercise pen. They will run about for a time and this is normal... but after a bit, the doe will usually let herself be caught.
Hope this is helpful. If there is one thing I have learned in the past ten years of country life, it is that it is easier and more productive to work with nature than to try to bludgeon it into submission. It is frustrating when animals do not do what we want and expect them to, but there is usually a good reason for it, assuming that our expectations are reasonable.