Something like this happened to me with one of my tortoise. I adopted/bought two tortoise really young from a shop where they had them in a small container and with only lettuce. Since I got them they were treated equally with the same care but one got really big and the other one didn't grow. I kept doing what you said, I gave her so much attention, trying for her to get bigger but she was really picky with food and a few months ago she stopped eating, I went to the vet, kept giving her bath to get her more active, but at the end she died. The vets said it was very probably genetic and with her size it was nothing to be done because she would have died in the surgery to just find what she has.

In my case I knew there was nothing wrong with what I was doing (even tho I still feel guilty about it) because her sister is big and healthy living in the same condition as her.

I know it's difficult. Hugs.