How to fatten my pup up?

My pup is getting pretty stringy. Being a puppy, she's not always very interested in her food. She'll eat some, but usually not all I give her. I'm starting to worry. What can I give her occasionally to put some meat on her bones that's not bad for her? ALSO, I'm not interested in changing her brand of food (that's not the problem). AND most importantly, she DOES NOT have some sort of parasite or sickness that's making her this way. Many puppies lack interest in their food until they're older, so I just want to know what I can give her to put some beef on her.