There are a few different things you could try. You coud try adding in a bunch more low sided(on multiple sides for easy access) litter boxes spread throughout their area. It may be with her disability that she just can't make it to their current litter boxes in time and needs extra to make it easier to reach more quickly.
If she's just being lazy and it's not due to having a disability, I think the only thing that could possibly work is restricting their area back down to a smaller area. Then if she is good about making it to the litter boxes after you do this, gradually start increasing the size of their area. If she is still having accidents in the reduced area, you may need to reduce it to a size that is small enough to fill up the floor completely with litter boxes. Then give it a week or two and remove one or two of the least used litter boxes and see how she does. If she is good about peeing in the remaining litter boxes, try removing another that doesn't seem to be used as much as the others. Then just keep doing that until there are only a few left. Then if it's all good, gradually start to increase the size of their pen until you eventually get their area back to normal.
The last option is if she is doing this due to her disability and extra litter boxes doesn't help. Then you may need to resort to covering the floor of their area with disposable pee pads and change soiled ones out as needed. I had to do this with an elderly rabbit that had some sort of degenerative spine disease. She gradually lost use of her hind legs and could no longer make it into her litter box. So I lined the floor of their pen with pee pads that I changed out as needed. Box of 100 at costco for $16.
If she's just being lazy and it's not due to having a disability, I think the only thing that could possibly work is restricting their area back down to a smaller area. Then if she is good about making it to the litter boxes after you do this, gradually start increasing the size of their area. If she is still having accidents in the reduced area, you may need to reduce it to a size that is small enough to fill up the floor completely with litter boxes. Then give it a week or two and remove one or two of the least used litter boxes and see how she does. If she is good about peeing in the remaining litter boxes, try removing another that doesn't seem to be used as much as the others. Then just keep doing that until there are only a few left. Then if it's all good, gradually start to increase the size of their pen until you eventually get their area back to normal.
The last option is if she is doing this due to her disability and extra litter boxes doesn't help. Then you may need to resort to covering the floor of their area with disposable pee pads and change soiled ones out as needed. I had to do this with an elderly rabbit that had some sort of degenerative spine disease. She gradually lost use of her hind legs and could no longer make it into her litter box. So I lined the floor of their pen with pee pads that I changed out as needed. Box of 100 at costco for $16.
