It sounds like either she doesn't believe you (not likely), doesn't care (possibly, depending on her personality), or doesn't know what to do about it (most likely). We aren't born with a natural knowledge of how to clean ourselves. We have to be taught. Also, it's possible she doesn't understand the importance of good hygiene. Obviously, it's easiest to teach this kind of thing with very young kids. (I'm not criticizing. I'm actually in the exact same boat with my daughter.) You'll need some strategy with older kids/preteens.
It's best to have these kinds of conversations outside of conflict. If you're in the middle of an argument, or she's particularly smelly, it's probably not a good time. Wait until you're both in a good mood.
I'd start off by acknowledging the fact that it's an awkward topic. "I know this is weird, and you're tired of hearing me talk about hygiene. But it's my job to make sure you know this stuff."
Offer options, which gives her some control of the situation. "Would you like to talk with me about it or would you rather watch a YouTube video without me around?"
As funny as it may seem to you, avoid offering sarcastic options: "Or, just keep stinking and have no friends." 1) This will undermine the fact that you're trying to help her. 2) It's probably not an option you're willing to live with.
If you think she'll respond well to it, take ownership of the problem. "I'm sorry I didn't teach you this stuff when you were younger." You can defend yourself if you need to. "This is my first time raising a teenager. I'm learning." But don't criticize. "I've been trying and you just haven't been listening!"
Make sure she understands the importance of hygiene or she won't care. It's not just about personal image. Talk about bacterial infections, fungus, and all the other fun things she might experience. Use examples from your life if you have them.