My puppy doesn’t eat as much as is recommended or gain much weight

I rescued a ~5 month old Australian Sheppard, Australian cattle dog, dachshund mix a month ago. She was 11lbs when I got her and now she’s 13lbs. She’s never eaten nearly as much as the dog food packages say. She has an open bowl of kibble in her crate 24/7. She gets like 2 wet food meals a day. I used to only fill her kong with wet food, but she stopped eating her kibble, so I started mixing the wet food into her kibble. She eats some kibble from her bowl in her crate, but never finishes it. She also has a snuffle ball filled that she used to LOVE but now avoids. I don’t want her to only eat wet food, so I think I’m gonna start soaking her kibble in broth. Should I consider switching to a different food puzzle?

The vet made a comment that she looks like the perfect weight a couple weeks ago. She looks a tiny bit skinnier now, but seems a bit taller, she still has a bit of a puppy belly though it’s just not as big. I just have no clue how to get her to eat the amount recommended and she’s growing slowly, but since we have only an estimated DOB and such a strange mix (no one knows how big she’ll get but the shelter estimates less than 20lbs) I’m so confused about if I’m doing anything wrong!! I constantly feel guilty and like I’m failing her. Does that feeling ever end? I’ve never had a puppy before. I feel like she’s a little behind the curve on training bc she was a rescue, so I always feel like I’ve wronged her by not being better about that. I am working w a professional trainer though so I have guidance. She also started basic obedience training a couple weeks ago, so she’s been eating a lot more treats, idk if that would impact her eating so much though

Also she had Giardia when I adopted her but she’s been treated and is fine now. She has to return to the vet for another shot soon, so I’ll double check she’s at a healthy weight again then, but I’m so scared the vet is going to think I’m starving her

I know 2lbs weight gain for a small breed is significant, but I am super concerned bc she doesn’t eat as much as packages say. She probably eats like 1/2-2/3 the portions recommended