Offering some hope...
For those of you who have had a puppy that is underweight and has consistent diarrhea - I have had that too. We took her to the vet 11 days ago and she was put on an antibiotic after being tested for parasites and any kind of stomach bugs. Nothing was found but we got the antibiotics anyway - needed to do something.
Dani was eating Diamond brand LBP food. Apparently something in it wasn't agreeing with her. She didn't do well the first several days. I slowly started switching her over to 4-Health brand and she's doing great! Salmon and potato is apparently the perfect food for her. Her stools are small and firm. Her coat is lustrous and she's very energetic. She gets probiotics still. And she is FINALLY gaining weight and growing! She'll never be a large dog and I'm ok with that but she's now a healthy girl.
I just wanted to give anyone out there that's having similar issues some encouragement. There is hope!