I have a 5 month old doberman girl that can not gain weight. She is wormed fully. Fecal check negative, and on heartworm prevention and suggestions?

You could try a different food, one that has a higher fat content.

The protein content in the weight gain foods is far too high for a young puppy.

As long as the pup is eating and is not skinny then it should be fine.

Check with your vet as to how underweight the pup is... remember it's always better for a dog to be slightly lean than overweight, and a lot healthier for them.