
Just a word of warning - dogs growth goes in 'fits and starts' - meaning they grow upwards - then outwards- then up etc.. Unless your pup is underweight I'd be really careful about trying to bulk her up as
too much weight on a growing dog can put a strain on her skeleton/growing bones, also it's possible to end up with an over-weight dog by over-feeding once the growing spurt has finished. Do make sure she's not thin though. Other reasons for lack of weight could be health related eg worms.
If you're really bothered by her weight I suggest you get her checked by your vet to make sure she's within a normal range for her size and age and breed.