Hiya Angie
we had the same problem with Poppy our wild wabbit baby!.
I decided to worm her (being a wild thing) and she gained weight thereafter..just a thought, but wild bunnies often have worms.
You do not always see these in the droppings as they are too high up in the digestive system.
As for feeding up, well we have started this routine with our elderly Sanctaury bunnies now ready for the Winter.
usual feed, but add ONE teaspoon finely rolled oats once a day (or twice a day in the coldest weather) and one teaspoon baby food (with the dried food, and oats) potato and carrot is good.
The Baby food helps the oats go through the system else the bunnies get constipated if they eat more than a teaspoon (ish).
Weetabix is okay for some bunnies, but we found it was too high fibre for some of our bunnies and made them bloated and unwell, so introduce anything new very slowly and carefully.
You could add some probiotics, eg, avipro in the drinking water as this will help stop the bunny getting an upset tummy with any dietary change.
Wild bunnies do tend to be on the skinny side, it took Hazel(our other wild rabbit orphan) five years to fill out, and he is now a well rounded wild bunny, but still incredibly lively.
He sleeps all day and comes out all night!!..does your doe have a similar routine :?: I would be interested to compare them
Best wishes
Adele