How much does she weight?

At three years, she should be on medium rats, or is she was a picky eater (really picky) large small rats.
I have two year olds, heading up on three on small rats, some on mediums.

Anywhere from 1000grams and up can usually take a small rat easily and start moving to medium rats.

Mice just won't cut it, and feeding more often....well, just switch to rats, and see how she is.