Do you know if she has any tumours? One of my first two rats developed a tumour, i was fairly young at the time, but I remember that as the tumour grew she lost more weight.
As for general weight gain, i've heard avocado is good as it's hight in fat and rats tend to find it super tasty. There are also products such as 'weight up baby' that are specifically designed for rats that have problem gaining weight. I'm pretty sure it's in powder form and you add water so it's a sort of mush. It comes in different flavours and has lots of nutrients and a good fat content, especially for older rats.