I’m sorry to hear your rabbit is having s tough time staying clean! A few things come in mind to figure out the best solution:
1) Is your rabbit able to clean him/herself, or is there a medical reason why he/she is no longer grooming herself? This can be an obvious disability or a more nuanced one, like depression that leads a rabbit to neglect caring for itself.
2) What is the litter box set-up and how often do you clean it? A litter screen has helped many rabbit feet, as it helps separate the feet from the soiled litter. See the photo album in my profile to see an example!
3) How would you describe your rabbits pee and poop, in terms of appearance and texture? Unhealthy droppings, like slushed pee and runny/watery poos are not only more difficult for a rabbit to clean off, but they are signs of poor health that need to be addressed.
4) Is your rabbit a long-haired breed where sanitary shaving may be needed regularly? Such as a lion head?
With some additional information, hopefully members can give some good solutions!
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.