Underweight baby?

Tamara

My 25 week old is 26 inches tall and 15 pounds. I was told she is too skinny for her height. She eats 6-8 ounces every 3 hours or so and also has infant cereal and either a fruit or veggie each day. I don’t know how to help her gain weight because if she doesn’t want to finish her bottle or eat more, she simply won’t.