Baby’s can be inconsistent, can’t they? It sounds like you’re doing a good job. I will say she is young to be getting veggies — When she’s older (5 months or so), you can better introduce veggies, but a baby rabbit’s gut environment is still developing. Introducing more complex things to digest (veggies) can be disruptive and lead to gas, discomfort, and indigestion. If possible, back down on the veggies.
In terms of the litter box, hormone development is probably going into play. Like a human baby, there are accidents, but for rabbits, usually the idea behind peeing/pooping is hormone driven — they are innately programmed to mark things that don’t smell like them. It makes the area more safe, because it’s “theirs”. For most people, you do what you can with litterbox training, but many rabbits do not become adherent consistently until they are spayed/neutered.
Keep it up, but know that if you’re hitting a ceiling or a digression in training, it’s probably hormones, which cannot be trained out unfortunately. She will need to be spayed once she’s old enough for this reason, as well as health reasons (very high risk of cancer in unspayed females). Now is a great time to establish rapport with a rabbit-experienced vet, if you haven’t already!
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.