If you're persisting with the bath, I'd try to figure out with her why she's not doing it and then get her to come up with some ideas for how to make sure she does it. So if she's forgetting, she might find a reminder sign helpful, for eg. It becomes her solution then, and she's more likely to actually do it.
The consequence, as you can discuss with her, for still not doing it, could be something like showers.